Complex sentence: Following the Soviet Union's withdrawal from Afghanistan in February 1989, Osama bin Laden returned to Saudi Arabia in 1990 as a hero of jihad, who along with his Arab legion "had brought down the mighty superpower" of the Soviet Union.
Split into simpler sentences: Following the Soviet Union's withdrawal from Afghanistan in February 1989, Osama bin Laden returned to Saudi Arabia in 1990 as a hero of jihad. Along with his Arab legion, he was thought to have "brought down the mighty superpower" of the Soviet Union.