If the Crusaders capture either Damascus or Cairo, what happens to Jerusalem?

Symbolically, Jerusalem is important. But either Damascus or Cairo are much more politically and economically relevant, so if the Crusaders capture either, would they shift their administrative centre from Jerusalem to either of those? Or would the lord they put in charge of Damascus/Cairo be forced to recognize the overlordship of the King in Jerusalem?
 
Damascus might have some issues geographically as far as defensibility goes.

But as with everything involving the Crusaders, the main determinant for things is simply who gets their first. Followed by lots of shouting debates about who gets new lands and who gets to keep the city.
 
Since I doubt the Crusaders can take Cairo or Damascus without serious outside help (likely the Byzantines, who if resurgent are a problem in and of themselves) it's hard to see this happening. But I'll overlook that for now.

Let's say the taking of Damascus in a more successful second crusade. More than likely, It will become its own principality like Antioch or Edessa (and unofficially Tripoil). Of course, this principality will be even more exposed than Edessa, and will likely fall very quickly, at which point I doubt the Crusaders can take it back, or will even want to.

As for Cairo, they would really need help from several pillars to take it: 1. A strong fleet, like that of Venice, Genoa or Byzantium, to help take the Nile Delta and supply; 2. A quasi-agreement with whoever is the dominant power in mesopotamia to not attack, because the Arabians sure as hell will; 3. Support from the local Copts, to keep densely populated Egypt under their control. 4. A general crusade with support from a large area like Germany or France

Under any of these scenarios for Egypt, the crusaders will be rather reliant on foreigners, and will probably have to give some key trade concessions and/or cities to their fleet help (Byzantium would love Alexandria, Genoa and Venice to destroy each other's trade quarters and grow their own). They will also have to deal with whoever the new crusaders from western Europe are, and what they will want. In the end, what you are likely to see is a small Kingdom of Egypt, ruled from either Cairo or Alexandria. If it remains Christian, this state will quickly eclipse Jerusalem due to Egypt's wealth and population.

In either case, these conquests are not going to end up as vassals to Jerusalem, but rather more as buffer states. The idea in taking Damascus or Egypt (in the 2nd and 4th crusades) was to create a buffer zone so Jerusalem itself would not fall.
 
The most plausible scenario for capturing either is probably a successful crusade, since I don't think the Crusader States on their own could raise enough men to take them. Given this, I'd expect that the new land would go to one of the crusader leaders as an extra kingdom.

I'm not sure what Damascus and its environs were like during this period, but if Cairo (and, I presume, Egypt along with it) were captured and given over for whatever reason to the Kingdom of Jerusalem, that kingdom's centre of gravity would shift pretty decisively to the Nile valley. In which case, I'd expect the main administrative seat to move to Egypt, with Jerusalem in use as a ceremonial capital for coronations, royal burials and so forth.
 
Amalric I of Jerusalem came pretty close to capturing Cairo more than once, and did hold Alexandria briefly as the Fatimid regime collapsed and the kingdom of Jerusalem and Nur-ed-Din fought a proxy war for control of Egypt. He didn't have the forces to hold Alexandria long term, but extracted a fortune from its inhabitants before returning to Jerusalem. I would expect a similar outcome with Cairo.
 
"Capital" supposes a single state, and the Crusader States are called that for a reason.

Since the whole point of the Crusades was the capture of the Holy Land, Jerusalem will be the official centre to any such conglomeration of polities. However, the states that use Cairo/Alexandria/whatever and Damascus are likely to have more power than the Kingdom of Jerusalem, even if they are nominal vassals.
 
But how would they get the manpower, I agree, the best case scenario would be at a point where the Fatimids are disintegrating, (1154-1169). Historically, Jerusalem under Amalric I worked together with the Byzantines under Andronikos Kontostephanos. However it was mistrust between the two that led to the breakdown of Crusader support for Egypt, and ultimately helped the Ayyubids in conquering Egypt.

Perhaps if Amalric wasn't so mistrusting of the Byzantines, it might have helped in not only securing a good deal of Coastal Egypt, but probably threaten to cut off the Ayyubids from Egypt.
 
Johnny K wanted to capture Damascus for the Crusaders and take Antioch for himself, so if Damascus was captured Jerusalem wouldn't be affected. However if the Crusaders held Damascus Syria would be cut off from Egypt so it would be almost impossible for these powers to combine under one ruler until Damascus had been retaken, which would take time and effort.
 
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