There have been a few attempts to apply IR theory to the Middle Ages, although I have to warn you it's not going to look like modern geopolitics in the sense of sovereign actors (states or otherwise) acting in their own interests. The general role of the Middle Ages in IR thought, at least recently, involves what the different sovereignty/political matricies of that era can say about the idea of geopolitics. (Naturally, actual medieval historians are more interested in the actual history.)

That said, you could check out Andrew Latham's Theorizing Medieval Geopolitics. I'm not that comfortable with some of the terms he throws around casually, but he's got it right that "state formation" is not a one-and-done creation of 1648, at least.