Divide and Conquer, or Divide and Rule, has been the axiom of ruling elites for millennia. Machiavelli wrote that, in the case of war, "A Captain ought, among all the other actions of his, endeavor with every art to divide the forces of the enemy, either by making him suspicious of his men in whom he trusted, or by giving him cause that he has to separate his forces, and, because of this, become weaker."
In Iraq, Divide et Impera meant encouraging the division of the country into ethnic based political parties, none strong enough on their own to topple the influence of the United States. The original Coalition Provisional Authority government of Iraq was picked entirely on sectarian lines - 12 slots for Shia, 5 for Kurds, 5 for Sunnis, and 1 each for Christians, Chaldeans and Turkomen. The post-invasion Iraqi national identity card identifies each person by their religion. The country was re-designed as a federation, and split largely along religious boundaries.
In India, for the British, it similarly meant playing pre-exisiting regional power structures against each other until they were politically incapable of uniting.
Domestically, divide and rule is a useful strategy whenever a small elite wants to maintain disproportionate power over the larger masses of the general population. The encouragement of sectarianism is a typical way in which to do this.
In the late 1960's this type of strategy gained a foothold in response to the mass protests of the civil rights movement and the anti-draft movement of the Vietnam War. The Democratic Party was particularly instrumental, beginning with their 1972 convention, of basing mainstream liberal politics around the idea of what has become 'identity politics'.
A generation later, the United States is more diverse, and more open towards diversity than probably ever before. There are college degrees ranging from African-American , to Asian-American, to Gay and Lesbian Studies. Mixed ethnicity relationships are generally accepted, and there is a president who is from a mixed 'racial' background. These are, in and of themselves, wonderful things.
But the United States is also country more economically unequal than at any time in the last 80 years. The labor movement is on life-support, at best. These conditions illustrate the limits of identity politics. Sectarian analysis disrupts the common class perspectives of the general population, to the benefit of a wealthy elite - though it may be an elite that is marginally more diverse than it was a generation ago.