So an area has a bunch of dragons. These dragons are intelligent, but instinctively solitary (that is, the way they feel about living in a group is analogous the way a human would feel about living alone) and therefore do not have a very integrated society. Usually, a single individual lives in a 'territory' and only interacts with others as necessary, although the concept of living in a group is not alien (again, roughly comparable to the way a human would think about living alone). Communication is not a problem.
How would these dragons be forced into organizing themselves to fight a war against a nearby nation of humans (medieval technology)? That is, what kind of threat would said humans need to pose to incentivize an organized response?
Note: I am not asking whether this scenario is viable. If you do not think it is viable, and have a reasonably similar alternative that is, please share it. Otherwise, please do not answer with some version of "they wouldn't."