I am also having issues with this mission, the first objective is to get to a city in time and defend it, you get there and that bit of the mission completes but then the second half of the objective doesn't complete, so you presume you have to stick around and defend and a big attack does come (along with many little attacks) but the mission does not progress. You can't capture the city and there doesn't appear to be anything you can do about it.
