OK, clearly the Aztecs are the obvious threat. I like the idea of a two-pronged attack, one at the capital and one at the Horse City. We could then cut them in half and cause massive unhappiness and chaos. Plus, that way if they have a powerful counterforce in one half of the country, we can contain it and still capture the other half before combining our forces to crush the resistance. But lets not forget about Tokugawa, I think it'll be in our best interest to stop the Japanese barbarians before they develope samurai.