Final answer:
The decline in defense spending between 1987 and 1999 was primarily due to the end of the Cold War.
Explanation:
The steady decline in defense spending between 1987 and 1999 can be best explained by the end of the Cold War. The Cold War, known for the arms race between the United States and the Soviet Union, led to a significant reduction in defense spending following its end in 1991 due to the dissolution of the Warsaw Pact and the collapse of the Soviet Union.
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