Complex sentence: In November 1989 both the United States and the Soviet Union declared an end to the Cold War causing relations between the United States and the Soviet Union to warm up, and in 1991 the two were partners in the Gulf War against longtime Soviet ally Iraq.
Split into simpler sentences: In November 1989 both the United States and the Soviet Union declared an end to the Cold War causing relations between the United States and the Soviet Union to warm up. In 1991 the two former rivals (the United States and the Soviet Union) were partners in the Gulf War against longtime Soviet ally Iraq.