I think that to make games like these enjoyable, you should be prepared for the fact that you can get attacked by anyone anytime in the game, and not just leave if you think you're going to lose or because you get surprised or whatever.
I can understand that it may not seem fun if you think you're going to lose. Well in this game I don't think the winner is determined yet at all so I guess that's not really the case here.
In any case... Leaving the game because it "doesn't seem fun anymore" after getting attacked is somewhat cowardly in my opinion. It makes the gameplay more boring, so I think we should consider some solutions to the problem.
1: Allow someone who isn't currently in the game to take control of Barbarian zones.
2: Set penalties for leaving the game that will apply the next game you join. For example, not being allowed to join the first round in the next game, losing 20% of your production the first 5 rounds, or something like that. There could be different penalties for leaving, giving your nation to someone else or getting eliminated (with no penalty for getting eliminated for example).
Those are some of the ideas I could come up with atm. If all playing people would be ok with it, I would be fine with taking control of Barbaria.