its called imperial overstreatch, when an empire cannot afford its commitments, or becomes over comitted by expansion and debt to the breaking point, and thus collapses from the weight of empire, like rome, byzantiuim, ottoman, russia, soviet union, ect.

its the USA's turn now to fall, thanks to crushing debt and too many wars, could the Libya conflict be the straw that breaks the camels back?

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

Leroy