Dozens of people were killed, all because a young boy crossed an imaginary boundary at 31st Street Beach.
To mark the 100th anniversary, residents gathered Saturday at the beach near a small marker noting where the unrest began. They held a ceremony, sponsored by the Bronzeville Historical Society, to bring to light one of Chicago's most violent racial conflicts.
In 1919, the 31st Street Beach was segregated. The violence began after a black 17-year-old named Eugene Williams floated beyond an imaginary boundary in the water. He drowned after white beach-goers threw rocks at him.
What followed on July 27, 1919, was one of the darkest moments in the city's history. During the riots, white mobs traveled through Chicago's South Side, beating blacks and burning their homes. Thirty-eight people were killed; 23 black, 15 white. Hundreds more were injured or left homeless.
"Even in Chicago today, we are so polarized by those same boundaries that existed in 1919," said Sherry Williams, the Bronzeville Historical Society's president.
WATCH: 107-year-old Red Summer survivor recalls terrifying race riots
At 107 years old, Juanita Mitchell, who was just 7 in 1919, has no trouble remembering the violence.
She had just moved to Chicago with her mother and sister and was staying on the South Side with her Uncle Cecil when the unrest began.
"I can hear my uncle saying, 'Here they come.' And when he said 'Here they come,' he meant the white folks coming down the street. And it was something," she recalled.
Franklin Cosey-Gay, executive director of the University of Chicago's Center for Youth Violence Prevention, said many of the attacks happened in what he called the "vortex of violence" near 35th and Wabash.
"You had a working, close tension that was boiling up," he said. "You had tension of not recognizing the humanity and hard work that blacks provided in the community."
The riots came as blacks flocked to the North for better opportunities and to escape Jim Crow laws in the South. Historian Timuel Black said the unrest helped to establish the Black Belt - the neighborhood we now call Bronzeville - where African American culture thrived.
"The riot was a stimulation as well as an example," he said.
Maya Barber was among dozens of teens commemorating the riots Saturday.
"Just because the color of your skin that shouldn't affect you being able to have fun and go to the beach," Barber said.
Andrew Tomachel, who attended the ceremony Saturday, called the Red Summer "a mark in our history that my ancestors were on the wrong side of."
"It's important that I acknowledge that," he said.