The Warsaw Pact was created in 1955. It was a military alliance of communist countries.
If one Warsaw Pact member-state was attacked, the other member-states would help defend it.
The original members were the USSR, East Germany, Poland, Hungary, Czechoslovakia, Romania, Bulgaria and Albania.
To control Eastern Europe. This can be seen as a response to the creation of NATO and the fact that Eastern Europe was surrounded by NATO countries.
West Germany was part of NATO and the USSR feared Germany was becoming strong again.