I'm trying to attach this territory, but it says there is a hostile army present.  There are no units here of any kind, and haven't been for many turns.  I've sent my army to patrol the entire place, through the fog of war, there's nothing here.  A scout eventually turned up and they chased it off, there are lots of diplomats here, but afaik they can't stop you attaching territories, and also there have been points when there have been none of them in the territory.  I'm guessing this is some kind of bug.

Eventually it worked, although I can't see what changed.

I *think* (not completely sure) that I've seen this happen when there are foreign units not just in the territory you are trying to attach, but when there are units in any territory already attached to the city that is attempting to attach the new territory.