The document summarizes key events that helped trigger the Second Red Scare in the mid-20th century in the United States, including the Soviet Union developing atomic weapons in 1949, the rise of Communist control in China, laws targeting Communist organizations and sympathizers in the US, high-profile spy cases involving the Soviets, President Truman's investigations of government employees for loyalty, Senator Joseph McCarthy's baseless accusations of Communist infiltration and his rise of McCarthyism, the House Un-American Activities Committee investigations of Hollywood, and McCarthy's eventual decline in popularity from 1954 after attacking the US Army.