Are you asking about misconceptions of the ideology itself, or of the geopolitics of Communism? Based on your last couple statements, I'll assume the latter.

First off, the word "imperialism" is loaded with political significance. I would hesitate to use that word unless your purpose is explicit condemnation of US policy at the time, which is, of course, your prerogative. But the grand strategy of the US during the Cold War can be traced back to George Kennan, who posited his theory of containment in the late 40s. Essentially, his argument was that the Soviet Union was an expansionist and ideologically driven empire with designs on global hegemony. The only way the Soviets could be stopped was through containment, which meant that the United States would commit itself to defending areas of broad strategic significance like Western Europe and East Asia. This is why the US involves itself so heavily in the Korean and Vietnam Wars, when the direct threat to US security is virtually nonexistent.

People like to talk about the domino theory as an explanation for American voracity in defending the corrupt South Vietnamese regime, and that's a fairly accurate metaphor. For US policymakers, it was far better to halt Communism in the jungles of Indochina than in other, more vitally strategic places like Taiwan, South Korea, or Japan. Preventing the first domino from falling would theoretically stop the spread of Communism in its tracks. However, this leads to the primary misconception of Communism for American policymakers at the time - Communism, and the Communist world, was NOT a singular, monolithic entity with fervent commitment to global revolution. To most Americans, it certainly seemed that way, particularly after the Communist victory in China and the subsequent denial of diplomatic recognition to the People's Republic. Looking at a map where the Soviet Union, Eastern Europe, North Korea, North Vietnam, China, etc., were all colored red, one could forgive the average American policy wonk for assuming that this was an inexorable spread of a singular ideological entity. Any spread of this contagion to Latin America, America's own backyard, would be easily interpreted as a direct existential threat to the United States, and would undermine US regional hegemony in the Western hemisphere. In order to preserve American power projection capabilities, it was - and is - necessary to maintain order in the region. When Communism reached Cuba, the quickness with which the US imposed sanctions, an embargo, and diplomatic isolation shows the seriousness with which it viewed any incursion into its area of influence.

Of course, US policymakers mischaracterized the Communist threat, as became increasingly clear after the Sino-Soviet split and border skirmishes between Communist Vietnam and Communist China. Nationalism was a latent factor in most, if not all, purportedly "Communist" revolutions, and many of the actual government economic policies implemented by these "revolutionary" governments had very little in common with Communist ideology. Even today, while China is nominally a Communist society and government, their economic system is undoubtedly far closer to Adam Smith than Karl Marx.

TL;DR version: American policymakers believed Communism to be a singular, unifying identity, one which trumped national borders and was capable of becoming a global ideology. This happened to not be the case, and nationalism ended up winning the day.