Currently playing as a Jewish ruler. My head of faith is defending against two wars - a claimant war for the Grand Theocracy of Israel, and a Catholic Crusade for the same title.
More than 390,000 Jewish troops have gathered for the Crusade. They all follow the war leader's army no matter what. Instead of defending against the crusade, the war leader goes off to fight the claimant.
390k troops follow him but they aren't allowed to join the fighting because they aren't in the claimant war. So they just sit around while he slowly loses a war on the aggressor's turf.
Meanwhile the Crusaders show up and conquer Israel. They would have been outnumbered 2:1 by the defenders, except the defenders have no interest in actually defending Israel. They just want to follow the high priest around and contribute nothing while he fights a totally unrelated war.
I wish there was a button for the player to select an allied army, no matter who the war leader is, and make them follow you.