The "God on our side" notion could also have come from the German army's belt buckle which, even in theologically unsound Uncle Adolf's time, bore the words "Gott mit uns". The German press incidentally nicknamed leading liberation theologian Ian Paisley "Gottes Feldwebel" (God's sergeant-major), which I suppose suggests that God should be pleased to have Paisley on his side.

But I don't know if the notion of "God on our side" really traceable to a single source, as the same general idea has been knocking around for at least a couple of thousand years.