One city challenge - They stole all my land

Had a game of One city challenge on immortal level, was cruising, or so I thought, then the polish who stole a city from my neighbour used Three great generals to steal all my land, felt helpless after that as was in no position to beat him militarily with my pathetic unit limit.

Is land stealing the most annoying tactic the AI can do? It is so annoying, what can I do to stop it.?