USA: Our Diversity Is Our Strength

Carthage:  Our diversity is our strength.
Rome:  Our unity is our strength.

1st Punic War:  Rome beats Carthage.
2nd Punic War:  Rome breaks Carthage.
3rd Punic War:  Rome destroys Carthage.

EDIT:  "Unity" was not exactly the right word for Rome; "cohesion" is more accurate, or perhaps "coherence".  Rome, and for that matter Alexander's Macedonians, had the cohesion and coherence that came from a shared culture and a shared identity.  Carthage and Persia, on the other hand, relied much more on adhesion and adherence.  Obviously those also worked well, or else we never would have heard of Carthage and Persia.  Adhesion and adherence, though, are much more likely to give way to the desertion and defection of whole groups of people, particularly in the tight spots when desertion or defection are most likely to be fatal.