Why was the United States government so afraid back in the 60’s of the Eastern world becoming full of communist countries?

The title speaks for itself. But I’d really like to know the answer to that question, because it seems like back then the U.S were extremely afraid of the Soviet Union, North Vietnam, China, Etc, and “Communism”.

IF every country over in the East would’ve converted to Communism, then what??? Would anything have changed for America, Canada, South America?

You always hear of the U.S fighting back in the day to keep Communism away, but you never hear of what would’ve happened [if] Communism would’ve took over in the East.