As told by Dave, Tarsh is a good reason.
In addition, I once played the 'River of Craddle' campaign with a Lunar Officer, whose family was on the loosing side of a dart war, and was 'promoted' to a new site: Head of Marine Imperial Infantry ... in Corflu. He went to Pavis through caravans, embarked on a boat to Corflu and the rest occured. And to explained why this lunar and various orlanthis and praxian cooperated: The enemy of my enemy is not automatically a friend, but is better than the full fledge enemy: Chaos. The guy that helps you destroy gorps and dragonsnails can not be completely bad (This is at least what decided Orlanthis and Praxians).