Edith Fowke and Joe Glazer provide this background information in their 1973 volume Songs of Work and Protest:

In 1931, coal miners in Harlan County were on strike. Armed company deputies roamed the countryside, terrorizing the mining communities, looking for union leaders to beat, jail, or kill. But coal miners, brought up lean and hard in the Kentucky mountain country, knew how to fight back, and heads were bashed and bullets fired on both sides in Bloody Harlan.
It was this kind of class war -- the mine owners and their hired deputies on one side, and the independent, free-wheeling Kentucky coal-miners on the other -- that provided the climate for Florence Reece's "Which Side Are You On?" In it she captured the spirit of her times with blunt eloquence.

Mrs. Reece wrote from personal experience. Her husband, Sam, was one of the union leaders, and Sheriff J. H. Blair and his men came to her house in search of him when she was alone with her seven children. They ransacked the whole house and then kept watch outside, ready to shoot Sam down if he returned.

One day during this tense period Mrs. Reece tore a sheet from a wall calendar and wrote the words to "Which Side Are You On?" The simple form of the song made it easy to adapt for use in other strikes, and many different versions have circulated.

Lyrics

Come all you good workers,
Good news to you I'll tell
Of how the good old union
Has come in here to dwell.

CHORUS:

Which side are you on?
Which side are you on?
Which side are you on?
Which side are you on?
My daddy was a miner,
And I'm a miner's son,
And I'll stick with the union
'Til every battle's won.
They say in Harlan County
There are no neutrals there.
You'll either be a union man
Or a thug for J. H. Blair.

Oh workers can you stand it?
Oh tell me how you can?
Will you be a lousy scab
Or will you be a man?

Don't scab for the bosses,
Don't listen to their lies.
Us poor folks haven't got a chance
Unless we organize.

Song originally issued on the Almanac Singers' album Talking Union (Keynote K 302A , July 1941, Pete Seeger lead vocal).