The question at the heart of The Cousins' Wars is this: How did Anglo-America evolve over a mere three hundred years from a small Tudor kingdom into a global community with such a cultural and linguistic hegemonic grip on the world today, while the other European powers - from Spain to Germany - did not.

The answer to this, according to Phillips, can be found in a close examination of the English-speaking people's three major internecine conflicts - the English Civil War, the American Revolution, and the American Civil War. These wars between cousins functioned as crucial anvils on which various religious, ethnic, and political alignments and successes were hammered out, setting Great Britain and America on a unique two-track path toward world leadership.