It's from the New York Times
Here's an excerpt:
Though the war in Iraq is one crucial cause, some students and experts on sectarianism also attribute the fissure to the significant growth in the Muslim American population over the past few decades.
Before, most major cities had only one mosque and everyone was forced to get along. Now, some Muslim communities are so large that the majority Sunnis and minority Shiites maintain their own mosques, schools and social clubs. Many Muslim students first meet someone from the other branch of their faith at college. The Shiites constitute some 15 percent of the world’s more than 1.3 billion Muslims, and are believed to be proportionally represented among America’s estimated six million Muslims.