The dramatic rise in defense spending in the first half of the 1950s could be best explained by the united states' involvement in

The dramatic rise in defense spending in the first half of the 1950s could be best explained by the United States' involvement in the Cold War.

Explanation:

The Cold War was a dispute between the Soviet Union and the Warsaw Pact countries against the United States and NATO. It began after World War II and lasted from about 1945 until about 1990. It came to an end as a result of the collapse of the Soviet Union. It was a "cold war" because there was no open war ("hot" war) between those countries.  

The Korean War, the Vietnam War and the Soviet Union's military intervention in Afghanistan were among the main scenarios of the Cold War. However, the main feature of the Cold War was not open combat, but competition for power and influence in the world. The two sides were spying on each other regularly.  

A major concern during this period was the rise of nuclear weapons and the danger of a Third World War that would destroy the entire planet. One of the focal points of Cold War tension was a divided Germany, especially Berlin which was a divided city on the border between the German Democratic Republic ("East Germany") and the Federal Republic of Germany ("West Germany"). The Berlin Wall divided Berlin into two parts - West Berlin and East Berlin - becoming one of the symbols of the Cold War.

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