It seems disingenuous to suggest that Communism did not present a threat to the United States during the 1950's and 1960's. The goal of the communist movement, as indicated in the Communist Manifesto, was the destruction of capitalism. To the Soviet leaders, the U.S. government represented the ultimate manifestation of communism. Most of the propaganda of the time suggested the destruction of the U.S. Had their been no real threat from Communism, then there would have been no arms race, no need for bomb shelters, and no discussion of "mutually assured destruction."
Most Americans saw the threat as not only invasion but also subversion. This threat was perhaps overstated, as many innocent people were accused of communist affiliation. So, infiltration may not have been a direct threat, but the threat of a nuclear war resulting in mutually assured destruction was very real. This threat would not have existed were it not for communism.
Soviet Marxist-Leninist Communism posed an indirect threat to the United States in the 1940's and 1950's after the Soviet Union had proved its viability by defeating Nazi Germany and imposing its domination on everything behind the Iron Curtain. The Soviets had no intention of trying to spread Communism to the United States, but they could spread revolutions all over the world and eventually leave the United States an isolated island of capitalism. Communism appealed to poor people, and most of the people in the world were poor. China became another Communist superpower under Mao Zedong. There were revolutions in Africa and South America. Each revolution seemed to lead to a Marxist-Leninist government, as it did in Cuba under Fidel Castro. The Soviet plans were not secret but obvious. Their plans were widely published in the Communist Manifesto. They did indeed pose a serious threat to the established order in the United States. The Korean War was Communist-inspired and supported by both the Soviet Union and China. The same was true of the Vietnam War in which the north was heavily supported by both the Soviets and the Chinese.
Decades old scholarship has since been updated by recently revealed documents about the Soviet Union.
One such study by the BDM Corporation asserts that the actual Soviet Threat was vastly over exaggerated by the U.S. The majority of the report comes from a declassified NSA report released in 2009. To quote the report;
"the defense industrial complex, not the Soviet high command, played a key role in driving the quantitative arms buildup" and thereby "led U.S. analysts to ... exaggerate the aggressive intentions of the Soviets."
There were of course monetary benefits to overstating the danger. Weapons, early warning systems, flash duck and cover films, sirens and so forth all bought at great expense. Another part of the report states;
"The Soviet military high command understood the devastating consequences of nuclear war and believed that nuclear weapons use had to be avoided at all costs."
It is possible to dismiss the report or slander it as revisionist propaganda, but the truth is that new finds and uncovered documents constantly update our view of history. It is important to stay abreast of new developments in the history field in order to keep ones knowledge current and correct.