Christopher Tyerman explains the innovation of the Crusades in the history of warfare.
The Crusades are distinctive wars, not because they’re religious wars – that’s common – not because they’re ideological wars, but because [of] the combination of violence and, what was offered by the popes for crusaders, the remission of sins. So this was the idea that to fight a war was not merely justified but actually was meritorious, spiritually meritorious; you got time off purgatory. To fight and die on Crusade, you went to heaven. So war was a penitential act, it was a good religious act like giving alms or going on pilgrimage or anything like that. And this was distinctive, this was different.