The counterculture of the 1960s refers to an anti-establishment cultural phenomenon that developed first in the United States and the United Kingdom, and then spread throughout much of the Western world between the early 1960s and the mid-1970s, with London, New York City, and San Francisco being hotbeds of early countercultural activity. The aggregate movement gained momentum as the African-American Civil Rights Movement continued to grow, and became revolutionary with the expansion of the US government's extensive military intervention in Vietnam.