Stupid question....How do I join a Crusade? The game is not clear on this, even though I'm sure it is very simple. Also, what determines the "contribution"? Is it just number of troops? How does the game actually determine/know his contribution?
Click the flag, it'll bring up the war page. Press the arrow to the pope's side.
Contribution seems to be based off of who killed the most enemy troops and who captured the most holdings. So theoretically a smaller power could still be the major contributor to the war by being more efficient.