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?