Complex sentence: Led by the United States at the insistence of US President Jimmy Carter, 66 countries boycotted the games because of the Soviet war in Afghanistan, though some athletes from some of the boycotting countries participated in the games under the Olympic Flag.
Split into simpler sentences: Led by the United States at the insistence of US President Jimmy Carter, 66 countries boycotted the games entirely because of the Soviet war in Afghanistan. Some athletes from some of the boycotting countries participated in the games under the Olympic Flag.