The French did exactly that, the ditch being the Maginot line. Those crafty Germans went through the forest though and with blitzkreig, once your first line of defence is broken, you're pretty screwed. Russia held out due to just being bloody massive.
I thought you might bring the Mongols up. I often wonder how effective they really were. Certainly they were very impressive in China, but their western conquests are less impressive than they would seem at first. They fought a Persia in serious decline, a completely divided Russia and couldn't get over the first western powers they came into contact with. Good, but I say the Germans, with their limited resources compared to the forces they were up against, were that bit better.