The army of the Kingdom of Jerusalem was made up of a few different groups: the “feudal levy” of all men in the kingdom (if they needed a full-strength army), knights from the military orders (the Templars, Hospitallers, and Teutonic Order), and crusaders visiting from Europe, if any happened to be in the kingdom at the time, whether it was a small handful of soldiers or a huge crusade.
We actually have a lot of very detailed information about the “feudal levy” in Jerusalem thanks to John of Ibelin, who compiled the laws of the kingdom (the “Assizes” or “Assises”). Thanks to John of Ibelin and the Assizes, historians often used to think that, whatever the definition of feudalism might be, Jerusalem was the best example of it:
“In the Assises of Jerusalem we have indeed the most perfect picture of the ideal feudal state, and they are themselves the most complete monument of feudal law.” (C.L. Kingsford, Cambridge Medieval History (Macmillan, 1926), vol. V, pg. 303-304, cited in La Monte, pg. 87)
So, as usual for questions about feudalism, it’s a good idea to read the posts in the FAQ first:
- Did Feudalism actually exist? by u/idjet
- When historians say feudalism never existed, what do they mean? by a deleted user
- The feudalism didn't exist AMA also by u/idjet
- The Recent Historiography of Feudalism by u/Miles_Sine_Castrum
"Feudalism” wasn’t a specific thing that the crusaders could bring with them and implant fully-formed in Jerusalem...but they did have a hierarchical system of mutual protection and benefits, where peasants worked on farms, local lords collected taxes from them, and so on up the hierarchy until the king. Is that feudalism? Eh...maybe.
Anyway, in Jerusalem there was a king at the top and four major barons - the Prince of Galilee, the Count of Jaffa and Ascalon, the Lord of Sidon, and the Lord of Oultrejordain (the Transjordan). Those baronies had their own vassals as well, so the Count of Jaffa was a direct vassal of the king, but his own vassals were the Lord of Ibelin, the Lord of Ramla, and the Lord of Mirabel. (The Count of Jaffa was also usually the Lord of Ibelin.)
These fiefs were a good way to organize an army when every able-bodied soldier needed to be called up. Each vassal owed a certain number of knights to the king. We even know how many soldiers were owed by each vassal - for example:
“The barony of the county of Jaffa and Ascalon, to which Ramla, Mirabel and Ibelin belong owes 100 knights:
Jaffa 25
Ascalon 25
Ramla and Mirabel 40
Ibelin 10”
(Edbury, pg. 165)
And so on for other vassals, for an army of 5000 knights, and (assuming they would each bring about 10 foot soldiers with them) maybe 50,000 soldiers in total.
At least…this is what John of Ibelin tells us. Unfortunately, it’s not really clear whether we can trust him. He was writing in the 13th century, long after the Kingdom of Jerusalem had mostly collapsed in 1187. His list refers to the sort of army that was raised (and destroyed) at the Battle of Hattin that year. Maybe this is also the size of the army that could have been raised (and, again, destroyed) at the only comparable battle in John’s time, the Battle of Forbie in 1244.
But in the mid 13th-century, there wasn’t even a king in the Kingdom of Jerusalem. The Holy Roman Emperor Frederick II had married the queen, Isabella II, who died giving birth to their son, Conrad. Conrad was legally the king, although he lived in Italy and never visited the kingdom at all. Frederick claimed to be regent of the kingdom for Conrad, but John and the other nobles in the kingdom thought they were getting along just fine without anyone from Europe bossing them around.
So, John was probably writing to prove to Frederick that Jerusalem already had its own customs and laws. No need for Frederick to show up! But was John accurately describing how the kingdom had functioned in the past, or did he and the other nobles just make it all up so Frederick would go away? What a coincidence that John described a strong, independent nobility…exactly the kind of government he would have preferred, instead of being ruled by an absentee child-king. So while we do have a ton of information about how an army was raised in Jerusalem, sometimes we have to be careful about who is telling us all that information, and why.
The kingdom’s army was supplemented by the military orders - the Knights Templar and the Knights Hospitaller in the 12th century, as well as the Teutonic Order in the 13th century. They were organized like monasteries but they governed their own big fortresses and smaller castles and churches, so they could call up an army in a similar way. There were the usual knights in armour and on horseback with the uniform we usually think of - white tunic/red cross for the Templars, black tunic/white cross for the Hospitallers, and white tunic/black cross for the Teutonic Knights. But they had foot soldiers too, called “sergeants”.
The military orders were technically independent and answered only to the Pope, so while it might be in everyone’s best interest to unite into one big army, in practice the Hospitallers and the Templars could act on their own. If the army of Jerusalem marched off to fight a Muslim army, the military orders didn’t have to come with them, and they often didn’t. They also went on expeditions by themselves without consulting the secular rulers of Jerusalem. In one case in the 12th century, the Templars attacked some Muslim ambassadors while the kingdom was at peace, and the king arrested them. Did the king have the authority to do that? Probably not! So there were competing interests and the military orders did not always cooperate with the rest of the kingdom.
There were also smaller military orders like the Order of Lazarus, which was for knights who had leprosy and couldn’t fight in a regular army. If you think that having a contingent of sick knights who couldn’t fight very well wouldn’t be a very effective way to win a battle…well you’re right! Almost all of the Order of Lazarus was killed at the Battle of Forbie in 1244 (but so were all the Templars and Hospitallers, and almost everyone else too, so that doesn’t really say much about the leper knights).
Lastly the army might be filled out by visiting crusaders. If there was a major crusade the armies would typically work independently under separate commands - so for example during the Second Crusade, there were really three armies: the armies of Louis VII of France and Conrad III of Germany, and the army of Jerusalem under King Baldwin III. In the 13th century, when Louis IX of France arrived, he took command of the whole army, since there probably weren’t enough men in the kingdom to assemble an entire army at that point. He and also took charge of repairing city walls and fortresses.
When a small force arrived it would join up with the army of the kingdom - so for example, the smaller groups of soldiers brought by Count Fulk V of Anjou in 1120, or the soldiers who came with Count Philip of Flanders in the 1170s. The army of Jerusalem could then conduct operations that they wouldn’t have otherwise planned on their own - for example when Philip of Flanders arrived, they joined together to besiege the Syrian fortress of Harim. The siege ended up being a failure for the crusaders, but they wouldn’t have been able to do it all without the Flemish soldiers.