Divide and conquer, or divide and rule, was a maxim Chinese strategist Sun Tzu stressed in 500 B.C. Sun wrote that the supreme military commander "subjugates other people's armies without engaging in battle, captures other people's fortified cities without attacking them." Defeating an enemy "without fighting is the true pinnacle of excellence." In the 4th century B.C., Macedonia's Philip II (Alexander the Great's father) demonstrated a genius for the type of duplicitous diplomacy that weakens and divides. The authoritarian Philip seeded resentment and aggravated old feuds in rival Greek city-states. Democratic cities were especially susceptible to his mix of threat,...