Total War: Warhammer III

If you have a military ally or a vassal, you can select them in the diplomacy screen (just select them, do not double-klick and go into the part where you can chooses things to negotiate about). There you should see three options in the middle bottom of the screen. The parchement for negotiations to the left, the X for leaving the menu to the right and the crossed hammers in the middle. Click that one in the middle. There you have four options a bit higher up. "Set Target", "Request Army", "Outpost" and "Ally Mission". Choose the second one and if you have enough allegiance for an army, you can take them over. Allegiance builds with time and certain events, but towards the end if you have had an ally for a while, you should easily be able to do that.