New Player Question --How am I Getting Flanked?

[Redfive]

I took both provinces back next turn but I had to use front line troops to do it so now the border is weakened and he will probably move in force to take some ground back.

It's a multiplayer game and I didn't get to play any turns Last night so I haven't progressed past reclaiming the initially stolen provinces.


So, to defend against this kind of attack I need to keep sizable forces/high prov. defense in all my territories?