Humans are tribal. We need to belong to groups. In many parts of the world, the group identities that matter most - the ones that people will kill and die for - are ethnic, religious, sectarian, or clan-based. But because America tends to see the world in terms of nation-states engaged in great ideological battles - Capitalism vs. Communism, Democracy vs. Authoritarianism, the Free World vs. the Axis of Evil - we are often spectacularly blind to the power of tribal politics. Time and again this blindness has undermined American foreign policy. In the Vietnam War, viewing the conflict through Cold War blinders, we never saw that most of Vietnam's capitalists were members of the hated Chinese minority. Every pro-free-market move we made helped turn the Vietnamese people against us. In Iraq, we were stunningly dismissive of the hatred between that country's Sunnis and Shias. If we want to get our foreign policy right - so as to not be perpetually caught off guard and fighting unwinnable w