Surprise attack...it'll take them some time to properly assemble for war against you.
Figure out, who the marshall is, as well as who the big-army lords are, and take them down first, before they assemble. Then start taking any available town or castle with your vassals. Even if you lose a fief or two, the AI won't be able to keep up with your speed.
And as long as you keep the marshall imprisoned, they won't gather at all, until they have chosen a new one. Take the sovereign down too, and they won't even select a new marshall.
The longer you wait (even if it's for buffering your garrison), the worse it'll be, because the AI lords will build up troops and feasts will reduce their infighting.
Another tactic is to leave the sovereign but beat up everyone else...after a few weeks, they might sue for peace. Then you can dishonorably break the truce again and start bashing away once more.
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And focus on lords, that have a good relation with you...once you take them prisoner, you can persuade them to join you. And make you sure you accept every lord interested in joining you, even dishonorable ones. Even if they're unhappy, as long as they're with you, they won't join the D'Shar.
