Trying to have a specific sort of match vs. AI

Long time lurker. Apologies if this is a dumb question. I've looked for answers but I'm not even sure how to phrase a search correctly.

When I play against the AI sometimes, I'll start with 1 AI ally on a ludicrous-sized map & trade & gather resources until there's practically nothing left, then change relationship to Enemy and battle. However, on Moderate, the AI seems to always declare war pretty early on, which I can handle, but it interrupts the peaceful nature of microing economic stuff. It doesn't turn on me on easier difficulties but it also doesn't build much of an army and it's always a slaughter.

So my question is:

I like doing only economic stuff & and then only military stuff, basically. How can I keep the AI from changing teams until way later (on harder difficulties) ? Or is this just against the fundamental nature of the game? Thanks in advance.

EDIT: I've also tried giving the ally tons of resources & telling him to build an army, but it doesn't seem to work very well.