Complex sentence: He campaigned in 1942 to expand the U.S. military commitment to a second front in Europe, a special interest of the USSR, which had ordered Communists throughout the world to promote the idea.
Split into simpler sentences: He campaigned in 1942 to expand the U.S. military commitment to a second front in Europe, a special interest of the USSR. It had ordered Communists throughout the world to promote the idea.