In Europa Universalis IV, how do I give orders to allied AI armies?

When in war together, my AI allies, colonies and vassals tend to do stupid things such as using their main fleet to blockade a single port, or right-out commanding their army home for tea time, all while my force is slaughtered by enemy stacks of doom. In situations where huge stacks are required, I often attach to an allied force in the hope that it will do something useful, but most of the time it is in vain. This is giving me a major headache, and I've lost countless wars to it.

So... is there a way to control allied armies and navies, for example by making them attach to my stack?

Internet research hints at the "Attach to this unit?" checkbox, but unfortunately it doesn't have a tooltip, and I haven't managed to get it to work. I've done some experimentation, and in the most successful case I got an army to follow me for two provinces before abandoning me. In all other cases, I was simply ignored.

Finally, once these basic issues are solved: Is there a way to transport allied troops over an ocean?

There's really no way to directly order around the AI armies, and you can't order them into any of your transports. The best thing to do is usually just to let them do their thing (save yourself the trouble of sieging the provinces close to your allies or rushing headlong into a doomstack) while you send your guys in to do the bulk of the work, like heading straight for the capital or harassing any new stacks that pop up.

With the Art of War DLC it is possible to give allied objectives which influence the actions of AI allies and subjects in wars. These are split into two types:

• Province objectives are handled from the province screen and allows the player to tell their AI allies to focus on specific enemy provinces. This will both make them more likely to send armies to besiege that province and to engage enemy armies that are located in it.
• Subject military focuses are handled from the subjects screen and allow the player to change the military priorities of their subject nations to either aggressive or supportive.
  • Aggressive subjects will not attach to your armies and will instead focus on their own operations, engaging enemy armies and besieging their provinces.
  • Supportive subjects will behave in the opposite fashion, attaching to or staying near friendly armies and avoiding independent operations in enemy territory.
  • If you have no military focus set, they will attempt to judge for themselves when it is best to attach and when it is best to act independently.