The war against india in the Renaissance era was not that hard, for some reason he had no units but garrisons stationed to defend his mainland cities while the rest of his army are stationed in a big islanseparated by a 1-tile sea housing 6 more cities of his,
The same thing happened with me in one war in my recent game. Carthage was split between a mainland and a nearby overseas colony, each having about 5-6 cities, and when I declared war she was caught with much of her army in the colony or moving through the ocean. I think maybe the AI has a tendency to overcommit its armies in service of offensive posturing, and then sometimes if its land isn't connected they're too far away to help defend.