How to make your vassal to fight rebels (with you)?

[GhostHobbit]

Playing Poland now, PU with Lithuania. LT has 20k rebels on their territory. LT has about 22k troops while I have only 15k. LT does nothing to fight those rebels. I want to help LT but no matter what I do they don't support me in fighting rebels (they start to move only when i'm already in a battle).

What would be the best strategy to make your vassal to combine forces and fight the rebels together?

 

[Beginner]

I have noticed this to. I think they need +4-5 troops over the rebels, since those were the only times I have seen them fight them (not really sure).
Didn't manage to find a solution when at peace. But when at war i put them at supportive so they attach my army and go stomp them.

 

[GhostHobbit]

Yep, at war there is no issue once u set your vassal into the supportive mode.