In my current campaign I was allied with the French for a long while, but tensions were growing with French troops massing on the borders. The French launched a crusade that would suck up my defenders from my front-line states so I said no Now we're at war but no land battles for a turn or two. The French sue for peace (of course) and I (foolishly) said OK and married a princess to the French. The naval war promptly resumed next turn (I should have known better).

Somehow, I'm not quite sure how, that damn crusade ends up on my land sucking up chivalric knights and I cant attack it

What can I do?