It should be noted when speaking about fears of radicalism that this was not entirely an invention. There were radicals within the United States and they did at times use violence. You can look backwards into the 19th century, towards moments like the Haymarket Square Riots (which featured an unknown bomb-thrower) to see that this would have already been a longstanding concern, and we can observe it also in the Red Scare itself, where bombs were being sent via mail.

At the same time, be aware that the idea of the international revolution was itself a major facet of communist ideology going back to Marx and that the Soviet Union did have its radical globalist wing. Trotsky in particular stands out as being probably the most famous advocate for the global revolution.

This being said, the Red Scare was a kind of hysteria, and one should not forget the xenophobia it displayed. Also, consider the way journalists and newspapers exacerbated these tensions and this confusion for the sake of a story. Periods of turmoil and instability tend to take on a life of their own, snowballing over time. Such was the case for the Red Scare.

There were several reasons why people feared a rise in radicalism after World War I ended. One factor was the arrival of communism in the Soviet Union. The people who supported the communist system, which is very different from the American political system of democracy and the economic system of capitalism, wanted to spread communism throughout the world. Many Americans believed that the Soviet Union was trying to bring communism to the United States. This was known as the Red Scare and had many people worried that radical ideas were gaining a foothold in the country.

Another concern was the growing number of immigrants coming to the United States from countries in south and Eastern Europe. These immigrants, some of whom were anarchists, had very different customs, languages, and ways of living than the Americans who had come to the United States from countries in north and Western Europe. Americans were frightened by these people's differences and wanted to limit their access to the United States. Many Americans feared that anarchists were connected to the spread of communism.

There were also economic issues after World War I ended. Inflation became a big issue, and many workers went on strike. These strikes, in some people’s minds, added credence to the idea that the communists were trying to spread their ideas to the United States. In some ways, it was easier to blame the communists than to deal with the economic issues that existed in our country.