even when you're the faction leader, your own faction forces you to make peace before you can make progress against another faction, and they're all a hivemind that vote the same way, to overrule them requires ridiculous amounts of influence
here's the repeating cycle:
1. faction declares war on you, they come at you with strong armies because they've been building up while at peace
2. battles happen, sieges are attempted, maybe some are successful, maybe they get reversed
3. when you've finally got them on the run, "Making peace with x" notification pops up, your faction forces you to make peace
4. all the enemy lords are freed, they begin to build up their forces again
then start over at step 1
and this cycle is even worse when you are one of the empire factions because the Khuzaits and Aserai like to come at you from behind while all the action is happening on the western frontlines, so they're able to grab a few fiefs, but then you arent able to wipe them out because your faction forces you to make peace when you finally get over there to deal with them