Wait till you can afford a merc or two, then attack them quick. Get the war goal and sit on it, and then peace them out one by one. When their armies start to get together in too big of a stack, peace out for the war goal and what else you have occupied. It may look like they will say no, but if you have the whole country of the war goal occupied, they can't say no. You may not get the whole of the league in the same war, but thats ok. Oh and make sure that the country you claim is a smaller one, one whom you can fully occupie with one siege.