4 ideas
In July 1099 the Franks took Jerusalem and massacred its Muslim and Jewish inhabitants. The officials offered sympathy, appointed a commission and did nothing. The episode shows a political center that could still perform grief but could no longer turn it into a military response.
The small Frankish forces won early because the Turkish and Arab emirs of Syria feared each other more than the newcomers. Emirs refused to relieve besieged neighbors, paid tribute to the Franks, and sometimes allied with them against fellow Muslims. An invader's real strength is measured less by its own numbers than by how divided the defenders are, and by whether the defenders treat it as one more player in their existing rivalries.
Zangi, Nur al-Din and Saladin revived jihad propaganda through preachers, poets and pious display. This gave them a moral claim that let them absorb rival Muslim cities, because resisting unification could be framed as betraying the faith. The ideology recovered Jerusalem in 1187, but it also served the rulers' own dynastic consolidation.
The Arab world expelled the Franks by 1291 but emerged weaker. Its politics depended on foreign military castes, on succession by assassination and fratricide, and on no stable legal limits on rulers. The Franks, meanwhile, carried Arab knowledge home and built institutions that let them keep learning. Winning a war does not decide the longer contest if the winner's governing structures cannot reproduce stability or absorb new knowledge.