IMO there should be only one army per kingdom led by the king. The king should have possibility to make detachments out of the army. As example, fast cavalry group to chase/harras smaller enemy groups, scout the area, do foraging => pillaging of settlements would get a reason to exist => we pillage to get our army fed. Could be done in a way player has it. Either by force or by persuasion/business.
On top of it, each army needs a war goal/target. What should be the target must be decided by war council. Another reason to have army camps in the game.
When player is a part of an army, he should receive orders from army commander. Not to relay/wait on rng ad-hoc blue exclamation mark that may or may not appear. This game needs more interaction of NPCs towards player.