Question

how did USA gain upperhand in the competition of stockpiling weapons

Answer 1

During the Cold War, the US and Russia built up extensive stockpiles of nuclear weapons in an arms race.
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Answer 2

After 1948, the Cold War started in a new phase.

It moved beyond the borders of Europe and America, and spread to Asia.

The USA and the USSR greatly increased their nuclear weapons.

There was a lot of pressure on the US government from its armed forces and arms dealers and manufacturers to increase its military spending throughout the 1950s and the 1960s.

As President Eisenhower was approaching the end of his term of office, he told the American public that too much power was there in the hands of the Military-Industrial Complex.

As an army general, he was able to resist the pressure, but he was also concerned that a future civilian president may not be able to resist the arguments for military spending.

Due to the Military-industrial complex nexus, the US gained an upperhand in stockpiling weapons.