Sam,

Are you looking at this from a COIN-insurgent view, or from a more conventional warfare view? WWI is a good example of a defense in depth, specifically looking at the Germans who were masters of it. Their defense in depth was so good that they had complete confidence they could hold the Western front with less troops (which they did for a long while), so they were able to shift more troops to the fight in Russia.

I recently read Decoding Clausewitz (short read, but well done IMO), and the author claimed that Clausewitz believed defense was a superior form of strategy than offense, and he considered people's wars/insurgencies to be defensive in nature. Worth reading if you are interested in defense. I was one of the guilty ones who relied on Clausewitz quotations, and after reading this book I'm now motivated to read On War (punishment for my sins).

Based on what I found interesting and relevant to our current wars I started a discussion on symmetrical defense strategies that may provide some food for thought.

http://council.smallwarsjournal.com/...356#post121356