Defence in Depth, with reference to British preperations for Sealion (twcenter.net)

Defense in depth was actually invented by the Germans in World War One:

http://en.wikipedia.org/wiki/Trench_warfare#World_War_I:_tre...

As were the concepts of Schwerpunkt and penetration. These are clearly described in, e.g., Rommel's book about his WW1 infantry operations (http://en.wikipedia.org/wiki/Infantry_Attacks).

Fortunately, that defense was never required - 70 years to the day since the famous speech:

"Never in the field of human conflict was so much owed by so many to so few."

http://en.wikipedia.org/wiki/Never_was_so_much_owed_by_so_ma...