Wikipedia note:

Seeger found inspiration for the song in October 1955 while he was on a plane bound for a concert at Oberlin College, one of the few venues which would hire him during the McCarthy era.[5] Leafing through his notebook he saw the passage, “Where are the flowers, the girls have plucked them. Where are the girls, they’ve all taken husbands. Where are the men, they’re all in the army.”[6] These lines were taken from the traditional Cossack folk song “Koloda-Duda”, referenced in the Mikhail Sholokhov novel And Quiet Flows the Don (1934), which Seeger had read “at least a year or two before”.[3]

Related Online

Husock, Howard.  “America’s Most Successful Communist.”  City Journal, Summer 2005:

The politicization of American pop dates from the 1960s, but it grew out of a patient leftist political strategy that began in the mid-1930s with the Communist Party’s “Popular Front” effort to use popular culture to advance its cause.

When BackChannels bolds an entry, as with the above, try clicking over for the joy of reading it.  I thought Howard Husock’s 2005 article a terrific piece of cultural, musical, and political history.  Pete Seeger passed away on January 27, 2014 (URL is to the obits).