I have a different strategy. It depends upon having the ability to renegotiate a peace treaty, and that you don't export any resources or luxuries to any AIs.
1. Sign an RoP with the target tribe.
2. Ship your troops and other units on over.
3. Once you have sufficient force, make sure you can trade for one of the AIs luxuries or resources, and make sure you have plenty of gpt. This might mean that you pillage roads on resources/luxury square, or it may mean that you gift cities to the target AI (with one unit right next to it so you can soon re-take it, of course).
4. Re-negotiate peace with the target AI. In the new deal you want a peace treaty, with gpt coming from you to the AI, and a luxury/resource from the AI to you.
5. Disconnect the trade route to your capital (no roads in the 8 squares around your capital seems to work best). This causes the target AI to declare war on you.
6. Attack and capture as many cities as you can hold.
Note that the large stacks of AI units generally don't consist of fast units. So, as long as you have a few decent units to defend your newly acquired cities, you may well not lose any cities on the first turn. But, what if the AI sends a massive stack of doom your way and comes next to a city very soon? Well, either raze it, or alternatively *gift* that city to another AI at war with the target AI. This will split up the stack, and give you more time for "target practice".
