Why did relations between the USA and the Soviet Union worsen between 1948-56?

In 1949, the prospect of further Communist expansion prompted the United States and 11 other Western nations to form the North Atlantic Treaty Organization (NATO). The Soviet Union and its united Communist nations in Eastern Europe founded a rival alliance, the Warsaw Pact, in 1955. The arrangement of nearly every European nation into one of the two opposing camps formalized the political division of the European continent that had taken place since World War II. This arrangement provided the framework for the military standoff that continued throughout the Cold. Europe was entirely divided and conflict between the Western nations (including the United States, Great Britain, and France…) and the Communist Eastern coalition began almost as soon as the end of World War II. The SOVIET UNION oversaw the installation of pro-Soviet governments in many of the areas it had taken from the Nazis during the war. In response, the USA and its Eastern allies sought ways to prevent further expansion of Communist influence on the European continent. In 1947, USA leaders introduced the Marshall AID Plan, a diplomatic initiative that provided aid to friendly nations to help them rebuild their countries from scratch. The discussions between the Western nations concluded on April 4, 1949, when the foreign ministers of 12 countries in North America and Western Europe gathered in Washington, D.C., to sign the North Atlantic Treaty. The most important reason for making it was as a security pact, which in one of the parts stated that a military attack against any of the countries which had signed the document would be considered an attack against them all. When USA Secretary of State Dean Acheson signed the document, it showed an important change in the American foreign policy. The formation of the Warsaw Pact was in some ways a response to the creation of NATO, although it did not occur until six years after the Western alliance came into being. It was more directly inspired by the rearming of West Germany and its admission into NATO in 1955. In the mid-1950s, the USA and a number of other NATO members began to advocate making West Germany part of the alliance and allowing it to form an army under tight restrictions. The Soviets warned that such an inflammatory action would force them to make new security arrangements, and they did exactly so. This all lead to relations between the USA and Soviet Union worsening because the way they were acting was very hostile to one another and would make the other side very worried about why they would act in the way they were acting.