What caused relations to deteriorate between the United States and the Soviet Union in 1979? The United States invaded Afghanistan. The Soviet Union invaded Afghanistan. The Soviet Union hosted the Olympic Games. The United States ratified the SALT II Treaty.
The best choice would be "The United States invaded Afghanistan". This made Soviet Union angry because it was them who fought in Afghanistan in 1979. Also, the Soviet Union is based on a communist system and they desire to spread it while the United States wanted to prevent communism.