• Hans Faber

Foreign Terrorist Fighters from the Wadden Sea

Capture of Damieta, Egypt

When it comes to the de-radicalization of foreign terrorist fighters, Mid and East-Frisians (in this context modern provinces Groningen and Friesland in the Netherlands and region Ostfriesland in Germany) do not have a good track record and have not much expertise to offer. The only thing they can contribute to today's challenges of combating terrorism is to illustrate its destabilizing and destructive effects in Frisia in the Middle Ages. We go back in time, to the era of the crusades.

1. Fighters off to foreign lands

A novum was created by the Roman Church at the end of the eleventh century: the Holy War. Not just your average regular war, but a holy one. A war to conquer the Holy Land. Creating the concept, frame or truth -whatever you prefer- of a bellum iustum 'legitimate war'. It was also a refined strategy of propaganda with the purpose of mobilizing knights and soldiers, and generating money to finance the costly endeavor a war simply is. Albeit being blessed, a holy war remains pricey. The whole thing meant a balance had to be found between the pacifist gospel on the one hand, and the need felt to defend Christianity with arms and violence on the other hand. No, not actually turning your other cheek (Matthew 5: 38-40). Crusades, therefore, are an example of complex planning and timing, communications and propaganda, of generating financial and human resources et cetera. To quote Tyerman (2015):

[medieval] Reason made religious war possible, a conclusion that might give anyone pause in the twenty-first century. Tyerman (2015)

Pope Urban II‘s successive call for battle and killing in 1095 against the Turks and the Arabs, and to conquer Jerusalem, had not been in vain. Crusade tax collectors, preachers and legates were send all over the lands to preach hate about Muslims, and to get as many volunteers to take the Cross as possible. Speaking less of the Commandment Thou shalt not kill and more of thou shalt kill. Back then, no entry visas were denied to these hate-preachers, to these Peter the Hermits. They traveled freely through Europe.

Pope Urban II's speech at Clermont to reclaim Jerusalem, 1095

The transformation of monastic orders of hospice into military orders, supported the crusades. The main orders were the Poor Fellow-Soldiers of Christ and of the Temple of Solomon (commonly Order of the Knights Templar), the Order of Brothers of the German House of Saint Mary in Jerusalem (commonly Teutonic Order), and the Order of Knights of the Hospital of Saint John of Jerusalem (commonly Knights Hospitaller). The Teutonic Order and the Knights Hospitaller had a strong presence in Frisia as well, although their houses and monasteries were mainly inhabited by slay-sisters. No knights therefore, since in Frisia no knighthood had developed anyway in the Middle Ages.


It all didn't end very well for the Order of the Knights Templar. Despite their sacrifices for Rome, it was king Philip the Fair of France, with consent of pope Clement V, who accused the Knights Templar of heresy on Friday October 13, 1307. French rulers always had a way with spending money, Confiscating the properties of the Templar helped to supplement the king's empty treasury. The Order was dissolved, many were tortured, killed and burned at the stake by the Inquisition. A big thank you for all their efforts during the crusades.


fresco from San Bevignate of the Knights Templar at the Battle of Nablus (1242)

The revenues generated from arable land and other assets in Frisia, of the houses and monasteries of especially the Teutonic Order and of the Knights Hospitaller, were taxed by their grand masters to finance the crusades. For example, in present-day province Groningen, region Ommelanden, prosperous cloisters existed, like the one in the village of Warffum. In the High Middle Ages we have a clear picture of this cloister. Its inhabitants were sixty nuns who were even led by a female prior. This house possessed ca. 2,250 hectares of both arable and grassland. During the crusades, the revenues of their lands were taxed to contribute to the wars.

Not only money was needed, also men. Cannon fodder, we would call them today. The concept of a holy war worked very well with the young men of Frisia. The Roman Church even spoke of the “Frisones et Teutonici” marking the specific contribution of the Frisians within the Germanic peoples. Again, meaning the Frisians from present-day provinces Friesland and Groningen, and of region Ostfriesland.

Not at first, but with the Third Crusade the pope’s call for battle got the full intended effect. Many Frisian men took the Cross. In modern terminology, many radicalized and became a foreign fighter. From 1097 until 1270 they fought in no less than seven crusades. Sometimes in Frisian maritime fleets of more than eighty so-called cog ships. Crusaders cruising the Mediterranean, but not on a holiday. They attacked, conquered, burned and sacked cities like Acre, Alcácer do Sal (Lisbon), Alvor, Antakya, Cadiz, Faro, Rota, Santiago, Silves and notably Tunis. Raiding the coast of the al-Andalus ‘Andalucia’ was a regular part of the itinerary when sailing to the Holy Land from the north, and in which the Frisian and Rhenish fleet played a big part too. These raids fitted within the greater scheme of the Iberian Reconquista.

During the Second Crusade, from 1145 to 1149, it were Frisians leading the way to take Lisbon, led by Popte Ulvinga (see further below). During the Third Crusade, from 1189 to 1192, landgrave Louis III of Thuringia gave the horse of a defeated Saracene emir to a strong and brave Frisian leader (Savelkouls 2016). Hence testifying of the participation of Frisians during this crusade too. And, of course, Frisians fought in Damietta in Egypt during the Fifth Crusade, from 1217 to 1221. Sparing in many cities no mosque and no Muslim's life, not even those of civilians. Concerning Tunis, they were Frisians from the regions Ommelanden and Ostfriesland who assisted king Louis IX during the Eighth Crusade in 1270.

Especially, the particiption and deeds of the Frisians in the Fifth Crusade is quite well documented in contemporary texts. These are primarely the De itenere frisonum, part of the Chronicle of the monastery of Bloemhof at Wittewierum in Frisia, the Gesta crucigerorum rhenanorum, and the Chronicle of the Conquest of Damietta by Oliver of Paderborn. Oliver was a German cleric, later bishop of Paderborn, and he was relevant in recruiting fighters from the various Frisian lands. Oliver probably did not travel via sea with the Frisian and Rhenish fleet to the Holy Land, but made the journey overland instead.

Converted to Christianity only relatively recent, apparently the opportunity to join an army and to fight again was a big pull for the young men of Frisia. Just as they had done before when volunteering the ranks of the Roman Imperial Army (read our post Frisian mercenaries in the Roman Army for this piece of violent history) to fight the Picts on the British islands. Later again, joining the fleets and war bands of their northern cousins the Viking (read our post Foreign Fighters returning from Viking war bands and be filled with horror) causing havoc and tears to many coasts of western Europe. Or, lastly, manning the English fleet of king Alfred the Great to fight the very same Vikings (read our post They want you as a new recruit).

The church knew exactly how to press the right buttons and spoke about "the honor of Christ that had been affected by the Saracens, i.e. Muslims from northern Africa, and the Mamluks." Or perfidious Saracens, in the words of Pope Innocent III (1161-1216). Honor was a very big thing, especially for the Frisians. A people still living in one of the last official feud societies of Europe. Of course, the full indulgences and booty promised by the church, contributed to their motivation too.

During the crusades, the Frisians were known for their impatience and eagerness to fight and kill. In the year 1214 a contemporary wrote:

Frisians would be ashamed if they would flee from the Saracens and therefore their only option was to kill or to be killed.

Stories have been passed on about Frisians leaving the battlefield to fight elsewhere when the crusade command stalled the attack too long. The common thread running through thirteenth-century texts about the Frisian fighters was their impatience, their speed in charging at the enemy, and their ferocity during battle. It were real Berserks or barbarians. See further below, at the end of this post, how Frisian fighters were equipped, and why indeed they were fast in charging at the enemy. In amente prevolans 'flying madly ahead' as it says of Frisian warriors in a warning of emperor Frederick I to his opponent the sultan of Egypt Saladin in the year 1188 (Mol 2002).

At the same time they were realistic about what was achieveable. When in the Fifth Crusade they were asked to take the trong city of Lisbon, the Frisians opposed to the Portugese and to the Dutch-Rhenish chapter of the crusade fleet, because they thought it would be too time-consuming both to take the great city and arrive in time in Italy. Fighting in the Levant was considered more priceworthy to them. Instead, the Frisian chapter left the fleet and continued to sail to the Medditerranean on its own, and successively raided the more manageable Muslim cities of Faro, Rota and Cadiz. The logic of the Frisian Alleingang might be explained by the fact they were participating primarely for their indvidual, personal benefit. To fulfill their vows. After all, high-medieval Frisia was a republican affair without having feudal lords to obbey.

In a way, to mark their barbaric heroism, the still existing Church of the Frisians in Vatican City was enlarged in the year 1141. To this very date its pious, Romanesque tower is the oldest in Vatican City, even of the eternal city Rome as a whole. Amen, indeed. Read also our post Magnus’ Choice: The Origins of the Frisian Freedom to learn more about the medieval history of Frisians in Rome.

coat of arms Roorda with Moor's head

Names of, partly allegedly, Frisian foreign terrorist fighters have been recorded: Aylva, Beyma, Botnia, Cammingha, Fatema, Galama, Hermana, Hettinga, Jarich of Hogebeintum, Thithard Jelgera, Joulsma, Dodo Kempinga, Lambertus of Katrijp, Liauckema, Martena, Ockinga, Popta, Popte Ulvinga (or was Popte, in fact, Hendrik of Bonn, a German knight?) and, of course, the famous Roorda van Genum. The coat of arms of the Roorda family still bears a black Moor’s head, as Roorda van Genum used to decapitate his opponents, so it is told. The same Moor’s head is depicted in the flag of Corscia. Check out our post Support of the Corsican Cause in Jeopardy to understand more about these macabre flags. But, among the crusade tales the most brutal of all was without a doubt Hayo de Violgama, also known as ‘Hayo with the flail’ (see image below).

legend of Hayo de Violgama aka Hayo with the flail

Hayo supposedly came from one of the many insignificant villages in rural Frisia, namely Wolvega, now in province Friesland. For the record, of course Wolvega is no longer insignificant. Others say Hayo came from the district Fivelgo in current province Groningen. Whatever his exact place of origin, legend has it Hayo had fought in the Fifth Crusade of 1217 and became famous for seizing the standard of the enemy during battle against the Saracens. Apparently standards were a highly valued commodity back then. By the way, in the year 732, Frisians fought in the Frankish Army in southern France against the Arabs, and one of these battles under the command of Saint Fris 'the Frisian' was named the Bateille de l'Étendard 'Battle of the Standard' (check our post Like Father, Unlike Son). Indeed, what is the deal with those colors, banners and flags Moreover, Hayo did not fight with a sword or spear. No, the trading mark of this fanatic was his flail he had taken with him from the farm (see image above).

And, it was Hayo who jumped as the first, or as the second since stories differ, onto the mighty tower of the city Damietta in the Nile Delta. Although the story of Hayo is pure fiction, the siege of Damietta by a large Frisian fleet is not. And no, the citizens of the city of Haarlem in province Noord Holland had no part in these fights at all, despite Netherlands’ history lessons at high schools learned otherwise until quite recently. Instead, Haarlem people were friendly law-abiding, nine-to-five citizens, earning a modest and honest living. Take that as a compliment, when reading this post.

Crusader Popte Ulvinga, also known as Poppo or Poptetus Ulvinga, deserves elaboration too. He came from the village of Wirdum in current province Friesland. In the year 1147, during the Second Crusade, crusaders took part in the siege of the Moorish city of Lisbon. The Moors were beaten. This victory is regarded as one of the turning points for the Reconquista of the Iberian Peninsula. The German knight and crusader Henricus of Bonn died during this battle. At the spot where he was buried, miracles occured. A palm tree on his grave was medicinal for whatever ailment people might have. When the remains of Popte were unearthed, they saw the palm tree had grown out from his heart. According to the journal of an unknown Frisian clergyman who traveled with the crusader's fleet to Acre in 1217-1218, this palm tree, in fact, grew on the grave of Poptetus from Wirdum. Poptetus had renamed himself as Henricus, also written as Hendrik, and was the commander of the Christian army and bearer of the standard. So, Wirdum and not Bonn.

the siege of Lisbon in 1147

The journal of the unknow clergyman was incorperated in the thirteenth-century Cronica floridi horti ‘Chronicle of [monastery] Bloemhof’. Monastery Bloemhof was located near the village Wittewierum in Frisia, of which Emo of Friesland was one of the founders. Abbot Emo of Friesland, by the way, was not a nobody but the first foreign student at the University of Oxford in 1190, among other.

Fighting the Cathars

Besides fighting in the Mediterranean and the Levant, Frisians also participated in the crusade against the Cathars in southern France, especially in Occitania (Moolenbroek 2021). The Frisian word ketter and German Ketzer, meaning heretic, derive from the word cathar. The civil war known as the Albigensian Crusade. Catharism was a religious, dualistic Christian movement between the twelfth and fourteenth centuries, heavily surpressed by the Catholic Church. Dualistic in the sense the Cathars believed in a feminine god. A monk of the monastery of Saint Pantaleon in Cologne, probably in light mood, reported that in the year 1212 many men from Saxony, Westphalia and Frisia went off to fight the heretics in southern France. The prior of the monastery Saint James in Liège, Belgium, reported that in total 30,000 Frisians went to fight in 1212. This number, however, is probably a little too much for being realistic. But, mistakes can be made, especially when you are in such a light and euphoric mood.

In 2010, the Frisians fought in a crusade army and took the strongholds of Minerve, near the great walled city of Carcassonne, and of Termes. In 1211, Frisians fought at Cabaret, Lavaur and Montgey. At Montgey, the crusaders were defeated and 5,000 Germans and Frisians were slain. Although, the tragedy at Montgey might also have been the slaughter of a 1,000 pilgrims, among them probably also Frisians. This slaughter of pilgrims from Frisia might have been an additional motivation for the Frisians to go berserk on the Cathars the following years (Moolenbroek 2021). In 1212, the Frisians successively participated in the sieges of the citadels of Saint-Antonin, Penne d’Agenais, Biron and Moissac. Also, in the year 1213 Frisians fought in Occitania, only unknown where exactly. During the siege of Toulouse in 1217-1218, the name of a Frisian cavalryman has been preserved, namely Rainers lo Frison. If you think Frisians and serving in the cavalry is surprising, it actually is not. Check our post Like Father, Unlike Son.

A high-medieval, epic poem in Occitan language, commonly known as Chason de la croisade albigeoise, written by William of Tudela (who lived around 1200) and an unknown writer, testifies of Frisians fighting against the Cathars. The list of regions of provenance in the Chason is long: Alaman e Bavier e Saine e Frison, Mancel e Angevi e Norman e Breton, Logombart e Lombart, Proensal e Gascon (‘Germans and Bavarians and Saxons and Frisians, men from Maine and Anjou and Normans and Bretons, Langobards and Lombards, Provençals and Gascon’). But also:

Aqui ac mot Frances e Norman e Breton, E i ac mot Alaman, Loarence e Frizon, E mot baro d’Alvernhe e mot ric Bergonhon

They are the French, Normans, Bretons, and they are the Germans. the Lorrains and the Frisians, and they are the barons of Auvergne and the powerful Burgundians.

Other Crusades

In the 1230s, Frisians also fought against rebellious Drents and Stedingers, i.e. the delta of the River Weser north of Bremen. In 1248, warriors of Tota Frisia also participated in the siege of Aachen (Mol 2002).

2. Fighters return from foreign lands

But all good things come to an end, eventually. At the end of the thirteenth century, after the last crusade, the Frisian foreign fighters too left the battle grounds of Palestine, Livonia, Prussia, northern Africa, southeastern France, Portugal and of southern Spain behind. For the tall big men of the north, as they were described together with the Danes that time, the fighting and glory was over. Those cruisiati who had survived, returned with their booty to Frisia.

Once back at their homeland, they did not have to fear prosecution by the redjeva or grietman ‘chosen judge annex prosecutor’ since what they did was not considered a (war) crime, yet. They were soldiers of a honorable Holy Army who had been fighting, killing and beheading Muslims, Latvians and Cathars, and raping their women, in legitimate wars. Back home, these veteran Frisian foreign fighters decorated themselves even with golden brooches made of Almohad 'Arabic' coins or copies thereof, to mark they had been fighting in the crusades in the Holy Land (Schokker 2018).

It was not over for the peoples living in the region we now call the Middle East, though. Almost eight centuries later, the Levant is still infected by ransacking, religiously motivated foreign terrorist fighters from all over the world, including some from areas that belonged to former Frisia. Even practicing the same cruel methods as the Frisian havedling (also Häuptling or hoofdeling) Roorda van Genum allegedly did.

The rehabilitation process of so many returning foreign fighters into the communal Frisian society became more than a concern. Absorbing this dangerous influenza went on for decades and decades. Nearly for two centuries. The Roman Church too, albeit slowly, became aware of the downsides of their successful radicalization programs. It was the patriarch in Palestine who in the year 1218 pointed out that crusaders had experienced great difficulties. Not only in getting to Jerusalem, but also afterward. That is no rocket science or apple sauce. You can imagine. These outlaws were radicalized, partly war-traumatized men who had to be fit back again into rural society after often three years of fighting in foreign lands. Home meant nothing more than a modest living in a hamlet and doing humble labor. No glory, no status and often no booty. Working the barren land in summer. Strengthening dikes to keep land and houses protected from the sea in autumn and winter.

Moreover, Frisian society was exceptional vulnerable for this deadly influenza as well. Frisia then, being a loose federation of free peasant republics stretching from present-day province Friesland to the region Ostfriesland in Niedersachsen. Unlike the rest of Europe, feudal structures were totally absent in Frisia. Therefore, de-radicalization programs could not be organized, if they would have thought about such programs at all. On top of the homecoming of killing-happy foreign fighters, the myth of the so-called ‘Frisian Freedom’ fueled anarchy. This freedom was granted to them by Charlemagne himself for their heroic deeds in battle in Rome. A myth which told that the Frisians were free from then on, and not subordinated to any lord other than the Holy Roman Emperor himself. They were Reichsunmittelbar. Read our post Magnus’ Choice. The origins of the Frisian Freedom for more information about this freedom. This way tipping over the already fragile balance between the free farmers republics and an anarchy, even further. The free republics were known as the the Seven Sealands under the treaty of the Upstalsboom (see further below).


The Republic TraditionNonetheless, the republican tradition of Frisia remained appealing and was picked up again with the foundation of the federation of the Dutch Republics at the end of the sixteenth century. Again a novum in Europe, next to the city of Venice. And February 1682, it were the States of Friesland, one of the republics within the Netherlands’ federation of Seven Republics, that voted as the second in the world, after France, to recognize the independence of the republic of the United States. Traditions in world history, when talking about republics: the Seven Sealands of Frisia, the United Seven Dutch Republics, the United States of America. Somewhere in between, a revolution in France marketing or selling ‘the republic’ as their noble contribution to the world. In fact, it produced Bonaparte soon after. Read more about the close ties between Frisia, the United States, and the Anglo-Saxon world in general in our posts Porcupines bore U.S. bucks and History is written by the victors – a history of the credits.


the Frisian Sea Lands during the crusades

Back to the High Middle Ages.

Reboelje ‘turmoil’ and civil war soon followed in Frisia. The fight between the Schieringers ‘Speakers’ and the Vetkopers ‘Fat-buyers’ started a few decades later, around 1300. Flails, pitchforks, shovels and alike were not solely working the barren land anymore. At first, relatively small scale and locally, but a century later it had become a full-fledged civil war with warlords (hovedling in Old Frisian language, haadlings in Mid Frisian language and Häuptlinge in German language) and armies from the Vlie in the Netherlands to the River Weser in Germany. A period known as the Great Frisian War. This weakness from inside, left Frisia prey for its hostile neighbors who would soon feast on it. Ironically, these Teutonic neighbors were supported by the same church and clergy that had praised the sky for the Frisian achievements during the crusades, not long before. The thanks they got.

The free republics of the Seven Sealands of Frisia tried to organize themselves within a collective defense mechanism called the Upstalsboom treaty. Similar to NATO seven centuries later. But it failed completely. Read our post Upstalsboom: why solidarity is not the core of a collective. Fascinating history, we think.

Thus, when hiking the Frisia Coast Trail and everything looks peaceful and is quiet, remember this used to be the Failed State at the Wadden Sea. Like standing on the shores of the Gulf of Aden today.

3. What did these warriors look like?

For those readers wondering what these foreign fighters must have looked like, please find below a mural from the church at Westerwijtwerd in province Groningen dating from the fourteenth century. It is thought to be a copy of a much older image dating from the twelfth century. Indeed, the period of the crusades. Its church is still standing, although serious earthquakes due to heavy gas mining do threaten it anno 2019.

fighters of Westerwijtwerd, fourteenth century

Typical features of the twelfth and thirteenth centuries fighters of Frisia were the heavy long, fish-skin like coats made of leather strips (called a schobbejak in Dutch language) and their heads shaven almost bold. The higher the hair was shaved off, the higher the status of the warrior.

...mith egge and mith orde and mith tha bruna skelde.

...with sword and with spear and with the brown shield. (First Riustringer Codex)

The weapons they were carrying were: a small round shield, a sword, called a truchslayn in Old Frisian language, and a long gaff annex spear. At the same time, this spear was used as a jumping pole to leap over the numerous ditches and small creeks in Frisia. The medieval images show that the spears had a disk or trident at the lower end. Just as for example the modern leaping poles, called a klootstock in Nordfriesland in Germany, or polsstok in the Netherlands being used to this day. Therefore, reckon these spears were at least 3.5 or 4 meters long. In German language this weapon is called a Sprungspeer. The warriors did not wear helmets, neither did they wear shoes. So, they were light and able to move and charge fast. ‘Naked’ as in unprotected, as they were described in the Middle Ages.

The long pole-spears bear resemblance with the well-known fifteenth-century Swiss pikes, but Frisians were using these spears much earlier. A report of the battle at Kuinre against Holland in 1397 illustrates one of its functions. It mentions the Frisians had driven their steel-tipped poles into a bulwark with the points facing upwards in a slant (Mol 2022). The trident made sure the spear would hold its position against the enemy.

Similar late-medieval murals exist in other churches in province Groningen as well, namely those of Den Andel, Stedum and Woldendorp. Walking barefeet was probably customary for the Frisians living at the salt marsh. A tradition that survived among the Frisians living at the Hallig islands in Nordfriesland even until the nineteenth century (Knol 2021).

an image of a seal of Upstalsboom treaty

The image in the church of Westerwijtwerd corresponds with many more images of Frisian fighters on several seals of the Upstalsboom treaty (read our post Upstalsboom: why solidarity is not the core of a collective) and of seals of Landesgemeinden or medieval districts like Rüstringen, Hunzingo-Oosterambt, Mormerland, Opsterland and Oostergo. It also corresponds amazingly well with the written observation we referred to earlier this post already, of a contemporary in Liège n the year 1212, when he was watching Frisian warriors going to fight a holy war:

Their hair shaved-off with only a tuft of hair left.

Another contemporary, Bartholomeus Anglicus, wrote in his De Proprietatibus Rerum, written in 1240, that the Frisians are really different from their neighbors. All the men have shaved off their hair. The higher their hair is shaved off, the higher their status.

How unique is all the above!


Note – Know that the crusaders Poptetus Ulvinga, Dodo Kempinga, Thitard Jelgera, and Rainers lo Frison will probably be indicted by the International Criminal Tribunal of Frisia (ICTF) in Aurich in Ostfriesland. Read the Press Release about these provisional indictments by the ICTF.

Further reading

Jansen, H.P.H. & Janse, A., Kroniek van het klooster Bloemhof te Wittewierum (1991)

Knol, E., For Daily Use and Special Moments: Material Culture in Frisia, AD 400-1000 (2021)

Lasance, A., Wizo van Vlaanderen, Itinerarium Fresiae of Een rondreis door de Lage Landen (2012)

Mol, J.A., De Friese volkslegers tussen 1480 en 1560 (2017)

Mol, J.A., Friese krijgers en de kruistochten (2001); Frisian Fighters and the Crusade (2002)

Mol, J.A., Vechten, bidden en verplegen. Opstellen over de ridderorden in de Noordelijke Nederlanden (2011)

Moolenbroek, van J., Friezen op kruistocht tegen de Katharen in Occitanië (2021)

Moolenbroek, van J., Nederlandse kruisvaarders naar Damiate aan de Nijl. Acht eeuwen geschiedenis en fantasie in woord en beeld (2016)

Nijdam, H., Lichaam, eer en recht in middeleeuws Friesland. Een studie naar de Oudfriese boeteregisters (2008)

Penning, Y., Emo’s labyrint (2010)

Pietersma, J. Friezen op kruistocht (1217-1221)

Readman, K., The Albigensian Crusade: Why did the Catholics start a civil war? (website)

Savelkouls, J., Het Friese Paard (2016)

Schokker, J., Insigne van een kruisvaarder? Over een pronkfibula gevonden nabij Uithuizen (2018)

Siefkes, W., Ostfriesische Sagen und sagenhafte Geschichten (1963)

Tyerman, C., How to plan a crusade. Reason and Religious War in the High Middle Ages (2015)

Villegas-Aristizábal, L., A Frisian Perspective on Crusading in Iberia as Part of the Sea Journey to the Holy Land, 1217–1218 (2021)

Wiersma, J., Noord-Nederland na de bedijkingen (2018)