The apotheosis of the trench: the Maginot Line.

Between the end of WW I and the beginning of WW II, at very great expense, France built a very large and very long super duper trench line that was called the Maginot Line. It ran from Switzerland to the Ardennes Forrest along France's border with Germany. If you look in the dictionary under "generals re-fighting their last war" you will see a picture of the Maginot Line. The problem was that this time around, the German's, using their new blitzkrieg doctrine, would simply go around the Maginot Line by coming through the Ardennes.

Oh, the futility! The French, at great cost in lives, finally figure out trench warfare and then build a giant, expensive monument to what they have learned, only to have it become immediately obsolete. You must learn quick if learning is to do you any good at all.