America's emergence as an active, operational world-power did not occur until the 1940s. During World War II and then beyond, America's industrial, military, and other capacities were finally evident as the strongest in the world. It was World War II which both marked and to a large extent caused this phenomenon.
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When the United States (and the allies) won the war and did a very large show of power when they dropped the atom bombs it emerged as a world power. The USSR learned the Americans had power and they meant to use it and were not going to let USSR bully them.
The conclusion of the Cold War was marked by the dissolution of the Soviet Union in 1991. This event effectively ended the ideological and geopolitical struggle between the United States and the Soviet Union that had defined the post-World War II era. The fall of the Berlin Wall in 1989 and the subsequent reunification of Germany also symbolized the end of the Cold War. The conclusion of this conflict led to a significant shift in global power dynamics and the emergence of the United States as the world's sole superpower.
A couple of events come to mind, part of the winning side in WW1 and WW2
World War I is significant because it marked the debut of many new types of weapons and was the first huge war to benefit from technological advances in radio, electrical power, and other technologies
Some of the results of the Spanish Armada were the Spanish economy was in trouble. The gold and the silver that Spain received from the Americas caused high inflation.It signaled the decline of great European power. It also caused Spain's economy to crash. This defeat proved that England had the power to defend itself. And in the end, England remained an independent and Protestant nation.
The first event was not in the U.S., but it is the test site of Bikini Atoll in the Pacific Ocean. Tests that were done in the 1040s and 1950s have left their mark--still radioactive after all these years. The accident at the Three Mile Island nuclear power plant near Middletown, Pa., on March 28, 1979 brought awareness of the dangers of nuclear power. Although there was no lasting environmental effect, communication to the public did raise fears.