Alliances are useful in some situations. In others,....

“ Alliances are useful in some situations. In others, they are absolutely vital.But they must always be approached with caution. Unity of that sort is based on mutual advantage. While that advantage exists the alliance may stand firm. But needs change, and advantages fade, and a day may come when one ally sees new benefits to be gained in betraying another.The warrior must be alert to such changes if he is to anticipate and survive an anannounced blow. Fortunately, the signs are usually evident in time for defense to be planned and executed.There is also always the possibility that changes will serve to meld the allies even more closely together. It is rare, but it can happen. ”

Timothy Zahn

From : Star Wars: Thrawn