The AI does a poor job of placing garrisons in key cities. For example, playing as the Confederacy in 1862 the Union AI left Harper's Ferry completely undefended: a key city with a depot and tons of supplies! This was playing with the AI on Sergeant/ Normal aggressiveness/ +1 activation.
In the near west AI Halleck's Army of the West in Missouri/ IT seems to wander around clueless... I realise it has to cover a large area but shouldn't it have some sort of strategy/ goals??