The Hyde Park neighborhood just south of the Black Belt, the site of the Columbian Exposition in 1893 and home to the University of Chicago, was one of the most intolerant neighborhoods. Property owned by a wealthy black businessman named Jesse Binga was bombed six times. Mary Bryon Clarke, another black homeowner in greater Hyde Park, had her properties targeted by bombs three times, even though the previous owners of two of her buildings had run them as brothels.

Such outbursts along the border of black and white Chicago fed into a general racial hostility. White newspapers resorted to dialect and minstrel-like scenarios to demean blacks and discredit their claim to housing and job opportunities. Black papers stressed a worsening climate of racial violence locally and nationally — often to sensationalized extremes — and denounced the unwillingness on the part of city authorities, especially the police, to protect African-Americans’ rights.

These factors did not make the 1919 riot inevitable. Once the conflict started, though, they made its escalation unavoidable. The worst of the violence — whites pulling blacks off street cars, hunting them down in the streets — were led by members of so-called athletic clubs, with names like the Hamburgers, Ragen’s Colts, the Sparklers and the Emeralds. These groups, funded and, when needed, protected by local Democratic ward bosses, were bent on revenging their party’s defeat in the mayor’s election that year.

Richard Daley, the future mayor and political dynast, was a member of the Hamburgers and allegedly took part in the violence — accusations that only burnished his reputation, later on, as a loyal neighborhood son willing to fight for the “integrity” of his turf.

As the riot spread, it followed the paths laid out by previous episodes of violence along the emerging boundaries of the Black Belt, or centered on contested areas where blacks were buying and renting property, but were not yet securely established in numbers. The generalized terror of the rioting emboldened whites to establish a racial quarantine of sorts, using sudden, brutal mob action to lay down a border between white and black Chicago.

For whites who hesitated, unfounded rumors of a black “invasion” moved them to action: Stories of blacks breaking into armories and preparing to “clean out” white neighborhoods to the west were taken as justification for pre-emptive strikes.

The police played a crucial role as well. During the first few hours of the violence, 2,800 officers, out of 3,500 total, were deployed along the edges of the Black Belt, forming a cordon. The police claimed they were separating the antagonists, but their strategy left few officers to patrol the rest of the city. The Tuesday morning rampage through the Loop, for example, took place while only two officers were detailed to cover the entire downtown.