You can’t just go to war.
I mean… I guess you can. But it’s not a good idea.
War takes work. It takes planning. It takes preparation. War, to put it simply, is a pain in the ass. You really have to want it.
And the workload only increases with the number of people involved. You would think that bringing the biggest army would make the task easier… and I’m sure it does make certain things easier, like flanking, reinforcements, enveloping, and that sort of thing.