Why did US attitudes towards the USSR change from 1917-1930's and then in 1941 when the U.S entered WW2?U.S history research

There was great fear of communism in the 1920s. 
Spearheaded by A. Mitchell Palmer, the red-scare was felt by most Americans.  Employers
were able to squash labor union strikes by insinuating the leaders were communists,
immigrants were expected to prove their loyalty, the KKK was able to take advantage of
the fears and grew to phenomenal numbers.

Hoping to balance
the growing power of the Axis countries, Roosevelt opted to acknowledge Russia as a
communist country during the '30s.

By 1941, Hitler had
turned on the Soviets and "our enemy's enemy is our friend" became the attitude of
politicians.  Better to befriend the communists in order to defeat
Hitler/Mussolini/Tojo.