Targaryen invasion

[SeanO]

I'm about ready to launch the Targaryen invasion. I'm a vassal of the north with several cities, keeps, and villages including King's Landing.

Will the Targaryens go to war against the north (or me) because I hold King's landing?  Do I get a chance to switch allegiances to the Targaryen?  What exactly happens when they invade.  I'd like to somehow convince the North and the Riverlands to either join Targaryens or ally with them somehow (as we've wiped out the westerlands and taken most of the Reach). Is this possible?