Since this seems to be a general strategy thread, I'll post my "invasion-of-rr'ed-territory-in-1-turn" strategy:
It is really quite simple.
All you need are howitzers, tanks and some controlzone igoring units
Use your controlzone igoring units to make a path up to an enemy city, and empty it with howitzers, then place 1 tank just outside it ready to take it over.
Simply do this with all his cities, and when all of them are empty, take them over..
This must, ofcourse, be done in 1 turn, but with the "magnificent" island fortresses the ai usually controls, that should not be a problem.
It is ofcourse fully legal to use your entire arsenal, just remember to wait with taking over his cities until all of them are "ready" for you
Pretty foolproof, except one little thing: If you do not take all his cities, you WILL have to take precautions to counterattacks and partisan pillaging