What organization did East Germany join?

Answer 1

The Warsaw Pact.

Although the USSR had been a valued member of the Allies in World War II, after the war the USSR was determined to expand its territory in Europe and Communist influence throughout the world. The "Cold War" began, an antagonism between the other three major Allies, Great Britain, France and the United States on one side and the USSR on the other, that never came to actual military conflict, though both sides prepared for the possibility. The three Allies and other Western European democracies formed the North Atlantic Treaty Organization (NATO) in 1949, pledging collective defense in response to an attack on any member by any external party.

Largely in response to the incorporation of the Federal Republic of Germany (generally known as West Germany) into NATO's mutual defense pact in 1954, the USSR formed its own mutual defense organization, the Warsaw Pact, in 1955. In addition to the USSR, the founding members were East Germany, Albania, Bulgaria, Czechoslovakia, Hungary, Poland and Romania. All of the members were Communist countries, and the governments of the smaller countries were controlled in whole or in part by the USSR. The Warsaw Pact enabled the Communist countries to stand collectively against NATO; specifically it allowed the USSR to maintain a protective buffer zone of satellite countries between it and the NATO countries. The USSR had suffered devastating losses when invaded during World Wars I and II during the previous four decades and was resolved to protect itself against any further invasions.

After several defections, the Warsaw Pact was formally disbanded in 1991 after the Cold War ended.