A pretty ignorant history question, but if two countries were allied, or in a faction together, would they still have armies bordering each other, or would they eventually redeploy to somewhere that is more of a threat?
I'd like to move some of my units around, but I don't want to leave a border entirely undefended, if it's a poor representation of what a country would actually do.
In my experience a unit or two may be left, but for the most part, no. However, I doubt the AI will care if you do move troops away.