1900's -- Labor organization
1910's -- Women's suffrage
1920's -- Scientific advances (Einstein, discovery of penicillin)
1930's -- The New Deal
1940's -- Fighting fascism
1950's -- The Cold War
1960's -- Civil rights movement
1970's -- Women's rights movement
1980's -- Gay rights movement and AIDS awareness
1990's -- Fall of communism and spread of democracy
2000's -- Digital Freedom (the "right" to hack a DVD)