Final answer:
The relationship between the U.S. and the Soviet Union after WWII was defined by the Cold War, an era of political and military rivalry that encompassed an arms race, including nuclear weapons, proxy wars, and a space race, without direct military conflict between the two superpowers.
Step-by-step explanation:
The political event that characterized the relationship between the U.S. and the Soviet Union for decades after WWII is known as the Cold War. This period of intense rivalry lasted from the end of World War II in 1945 until the dissolution of the USSR in 1991. The Cold War was marked by a competition for global supremacy that included an arms race, including nuclear weapons, proxy wars, espionage, and a struggle for ideological dominance. The two nations emerged as superpowers with opposing economic and political systems, dramatically influencing world politics and military strategies. Although direct military confrontation between the U.S. and Soviet forces did not occur, the conflict was evident in various arenas such as the space race, political assassinations, and the formation of international alliances.