Final answer:
The Cuban Missile Crisis marked the first significant direct conflict between the U.S. and the Soviet Union during the Cold War, which led to increased nuclear arms development and several proxy wars. The intense rivalry had global implications and was central to the bipolar world order of the time.
Step-by-step explanation:
The first significant conflict between the United States and the Soviet Union during the Cold War occurred with the intense confrontation over Soviet missiles in Cuba, known as the Cuban Missile Crisis. This event culminated in the establishment of a direct hotline between Washington and Moscow which underscored the perils of nuclear escalation between the two superpowers. Following this crisis, both the U.S. and Russia continued to develop their nuclear arsenals, recognizing that the doctrine of mutually assured destruction (MAD) was central to preventing outright war. Additionally, the Cold War rivalry expanded into various international arenas, with proxy wars in places like Vietnam and Afghanistan where each superpower supported opposing sides. The Cold War period was marked by a bipolar world where the U.S. and Russia vied for global supremacy, influencing foreign policies and leading to a worldwide security dilemma.