Through lovely hypocrisy. Isn't that what all international politics is based upon? 

Its mostly because they both have ambitious leaders that the dynasty and empire still quarrel. We supported Sep's design process both because we like such and because empty parts on the map roughly where numerous PCs are headed can be inconvenient. 

Oh, and there's a short non-mountainous border between those two nations. Both sides heavily fortified. We even named the no-man's land in between. "the field of bones"
Tens of thousands of people from both nations have died there over the centuries in pointless border clashes, with neither side gaining much of anything.