Final answer:
Stalin's backing of communist parties in Eastern European nations like Poland, Czechoslovakia, Hungary, and Romania led to the formation of NATO in response to Soviet expansion.
Explanation:
Joseph Stalin's actions in backing communist parties in Eastern European nations like Poland, Czechoslovakia, Hungary, and Romania led to the formation of NATO. The establishment of communist governments in these nations prompted the United States to respond by forming the North Atlantic Treaty Organization as a defensive alliance against Soviet aggression.
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