Do you think Americans were justified in their fear of radicals and foreigners in the decade following World War I?
The war itself was decided on the battlefields of France and Germany. The French saddled the German republic with so much debt that it had no hope of regaining power, at least through normal channels.
But in 1917 when the Bolshevik war broke out in Russia and Russia exited the hostilities of WW1 a long simmering fear came to fruition. From the time Karl Marx published his treatise people became wary of those who called themselves socialists, radicals, and anarchists. And for the most part, those groups satisfied their interests within the confines of Europe.
America's lone foray down that line was with the Industrial Workers of the World, a socialist union oft used by radicals and anarchists for reasons other than unionization. But following the Lawrence (MA) Textile Strike in 1912, the IWW failed to gain much interest with American workers. Membership declined steadily until its power over any group of laborers was minimal at best.
From 1919 to 1941 America was in an isolationist mode feeling that the two great oceans which exist on its coasts plus a strong internal police force would keep such groups at bay.