the muslins destroyed half of christemdom, getting countries piece by piece, for centuries.

most of the muslim world west of Arabia/Persia was christian before.

even switzerland was briefly sacked by muslims from Fraxinetum (near Marselle). Rome was fully sacked.

there were no international organizations, no UN or NATO, the only supranational organization was the Church -

which took centuries of bad news until it tried to organize a response.

actually, some french nobles were going to Iberia to fight muslims - non-inheriting nobles, mostly, hoping to carve a county for themselves instead of being the brother of the Duke. The first good news for centuries, coming from long distance collaboration.

after centuries of seeing churches destroyed, people reduced to slavery and dhimmitude.

After Manzikert, the Eastern Romans asked for help, even if they were schismatics.

then the pope went to France to push the french idea further.

make the nobles swear not to attack the feufs of the crusaders while they are away (a very important promise even if not always fulfilled).

make them swear to take the cross, build an army, spend sometime there.

the crusades were not successfull in the long term, but they mounted a defense bringing the fight to the mediterranean. No muslim polity in France afterwards, mediteranean islands recovered, etc. To achieve that, the divided kingdoms needed some supranational help that only the Church could provide.