While Sunni and Shiite Muslims are divided by deep doctrinal differences, Sunni and Shiite extremists have always been united in their hatred of the U.S., and in their desire to bring it to destruction. The majority of Muslims does not share that hatred. The U.S. is the country most visited by Muslim tourists and it attracts the largest number of Muslim students studying abroad. But to understand the extremists, it is important to set aside the Sunni-Shiite divide and focus on their common hatred of America.
The suicide attacks that claimed the lives of over 300 Americans, including 241 Marines, in Lebanon in 1983, were joint operations of the Khomeinist Hizbullah and the Marxist Arab Socialist Party, which was linked to the Syrian intelligence services. The Syrian regime is Iran's closest ally, despite the fact that Iranian mullahs regard the Alawite minority that dominates it as heretics.
There are no Palestinian Shiites, yet Tehran has become the principal source of funding for radical Palestinian Sunni groups, notably Hamas, Islamic Jihad and half a dozen leftist-atheist minigroups. Hamas leader Ismail Haniyeh refuses to pray alongside his Iranian hosts during his visits to Tehran. But when it comes to joining Khomeinist crowds in shouting "Death to America," he is in the forefront.
With Arab oil kingdoms no longer as generous as before, Iran has emerged as the chief source of funding for Hamas. In short, the claim that al-Qaeda and the Khomeinists would not work together because they have theological differences is both naive and dangerous.