What follows is a very general outline of the issue.
It's important to bear in mind that the red scare of the 40s and 50s is the Second Red Scare. The First Red Scare emerged in the aftermath of the Great War. The First Red Scare’s emergence in 1919-20 came hot on the heels of the Wilson administration's severe abrogation of civil liberties – freedom of dissent being a good example – during the Great War. Under Attorney General A. Mitchell Palmer there were the so-called Palmer Raids. These were aimed at rooting out and imprisoning communists, socialists, and anarchists (still some of them around!). Some of those imprisoned did harbour radical views, but many caught up in the raids were simply trades unionists, leftists, or those whose beliefs were largely harmless but were considered ‘un-American’. US anti-communism can in some ways be connected to wider anti-collectivist tendencies in American politics from Thomas Jefferson onwards. Anti-collectivism was a distinct thread in American political thought – a member of a collective cannot be a citizen within the republic (which helps to explain not just anti-communism, but also - to a certain extent - anti-Catholicism). The newspapers were full of stories of plots to bring communist revolution to the US, bomb scares (real and imagined), and all sorts of harum scarum stuff about existential threats to the republic. In short, there was a degree of public fear and concern about communism, but there were also many who were concerned about the abrogation of civil liberties outside of wartime, etc.
At the same time as the Palmer Raids are taking place, US forces are fighting against the Bolsheviks in the Russian Civil War (1917-1921-22). Allied (US and UK) intervention in the Russian Civil War on the side of the Whites was founded in concern over Tsarist Russian war debt, anxiety about the spread of revolutionary ideas (note that Western Europe – esp. Germany – is in a bit of a ferment at this time) and other matters. As a side note, this created of persistent sense for some Bolsheviks that the United States and Britain attempted to kill the revolution at birth.
Then again, as the Great Depression took hold in the 1930s, many looked to the USSR (whose planned economy suffered slightly less from the ravages of the period) and communism as a positive force. Writers, journalists, politicians, and ordinary citizens were frequently amazed at the marvels of Soviet progress, from the orderly housing blocks to the gigantic infrastructure projects such as canals, hydroelectric dams, and vast steel works. The depression in part explains why so many Americans were, in the 1930s, attracted to communism and joined communist groups, actions that many would bitterly regret with the emergence of virulent anti-communism in the 1940s.
Between the 1930s and onset of the Second Red Scare, we've obviously got World War Two. The Molotov-Ribbentrop Pact of 1939 obviously provoked concerns that the Nazis and Soviets were cosying up to each other. Yet when the Grand Alliance was formed, the USSR, its citizens, and even Stalin himself were portrayed in an extremely positive light in propaganda movies, Hollywood films, periodical articles, and so on. BUT, in 1940 there is passed by Congress the Smith Act (properly known as the Alien Registration Act) which for the first time targets communists (and anarchists and fascists) who are thought to be advocating the overthrow of the US government.
Hope this helps. Happy to expand on any of the above if required.
Malcolm
Useful reading on this topic
David S. Foglesong, America’s Secret War Against Bolshevism: U.S. Intervention in the Russian Civil War, 1917–1920 (Chapel Hill: University of North Carolina Press, 1995)
Richard Gid Powers, Not Without Honor: A History of American Anti-Communism (New York: Free Press, 1997)
Norman E. Saul, Friends or Foes? The United States and Russia, 1921–1941 (Lawrence: University Press of Kansas, 2006)
__________War and Revolution: The United States and Russia, 1914–1921 (Lawrence: University Press of Kansas, 2001)