Allies don't go to war with each other most circumstances. And the phrase 'a common defense ...' cuts a pretty narrow slice of history, like only the 'NATO' countries? They have a lot more in common that just capitalism.
It may be a narrow slice of history but it is a true slice. The wars that have caused death of millions arise from:
a. Religions fighting religions or everybody else.
b. Socialists fighting everybody else or occasionally each other.
Secularism has NOTHING to do with it. Prager is simply wrong.