The Warsaw Pact was established in 1955. It was a military alliance of communist countries. It relied on the theory of collective security – if one Warsaw Pact member state was attacked, the other member states would help defend it.
This can be seen as a response to the establishment of NATO in 1949 and the fact that the Eastern Bloc was surrounded by NATO countries.
West Germany was allowed to join NATO in 1955. It was also permitted an army and air force - the USSR saw this as a threat that could lead to a new and strong Germany.
The Warsaw Pact was dominated by the USSR. This allowed the Soviets to force their foreign policy on the rest of the Eastern Bloc.
From 1955 Europe was divided into two armed camps - the frontlines of the Cold War had been established.