On May 4, 1886, a deadly confrontation between police and protesters erupted at Chicago's Haymarket Square. A labor strike was in progress at the McCormick farm equipment works, and police and Pinkerton security guards had shot several workers.

A public demonstration had been called to protest police violence. Eyewitnesses later described a "peaceful gathering of upwards of 1,000 people listening to speeches and singing songs when authorities began to move in and disperse the crowd." Suddenly a bomb exploded, followed by pandemonium and an exchange of gunfire. Eleven people were killed including seven police officers. More than a hundred were injured.

The Chicago Tribune railed against "the McCormick insurrectionists." Authorities hurriedly rounded up 31 suspects. Eventually, eight men, "all with foreign sounding names" as one newspaper put it, were indicted on charges of conspiracy and murder.

No evidence tied the accused to the explosion of the bomb. Several of the suspects had not attended the rally. But all were convicted and sentenced to death. Four were quickly hanged and a fifth committed suicide in his cell. Then, the Illinois Governor, Richard Ogelsby, who had privately expressed doubts "that any of the men were guilty of the crime," commuted the remaining men's death sentences to life in prison.

Illinois's new governor, John Peter Altgeld, pardoned the three surviving men. A German born immigrant who had enlisted in the Union army at the age of 15, Altgeld declared, "The deed to sentencing the Haymarket men was wrong, a miscarriage of justice. And the truth is that the great multitudes annually arrested are poor, the unfortunate, the young and the neglected. In short, our penal machinery seems to recruit its victims from among those who are fighting an unequal fight in the struggle for existence."

After granting the pardon, he said to the famous attorney Clarence Darrow: "Let me tell you that from this day, I am a dead man, politically." There was an immediate outcry. The Washington Post asked rhetorically: "What would one expect from a man like Altgeld, who is, of course, an alien himself?" The Chicago Tribune stated that the governor "does not reason like an American, does not feel like one, and consequently does not behave like one."

In 1889, the American Federation of Labor delegate to the International Labor Congress in Paris proposed May 1 as international Labor Day. Workers were to march for an eight-hour day, democracy, the right of workers to organize, and to memorialize the eight "Martyrs of Chicago."