What factors led to the red scare and mccarthyism in the 1940s and 1950s?

The "outside" factors causing the 'red scare' were: a) the development of the atomic bomb by the Russians, b) the establishment of Communist regimes in several East European countries by the USSR, c) the take-over of mainland China by the Communists and d) the attack of Communist North Korea on South Korea, leading to the Korean War.

The "inside" factors within the USA for it were a number of espionage trials against people suspected of working for the USSR. And generally speaking, conservative politicians and their financial backers in the industrial world had already a long tradition of seeing pro-labor laws (like the abolition of child labor and maximum working hours) and even the New Deal as so many communist-inspired plots against the Free Market and the American Way.

The factors leading up to McCarthyism were: a) President Truman's Loyalty Review program that he instituted in 1947 under Republican pressure to check on public servants and which program was expanded under President Eisenhower who limited the possibilities for defense and appeal against accusations; b) the institution of the House Committee on Un-American Activities (which in turn led to a great number of State and even private 'review' Boards on the subject. And finally there was the FBI under the violently conservative and anti-Communist Edgar J. Hoover who saw to it that people could be accused of being Communist sympathisers without the victim being told who was accusing him and of what , exactly.