Was it because returning soldiers were more quick to use violence? Or did the poor economy (itself a result of the government mishandling the reintegration of thousands of soldiers back into civilian life) cause white Americans over and over to choose violence out of fear that Black Americans were going to take their jobs?
And what about the association with Blacks and Communists? I have always been told the "Red" refers to both communism and the blood that was spilled.
Or was there something else there? Was the broader society just more tolerant/used to violence in general due in part to the extreme death toll of WWI?
To be certain, the roots of the explosion of the collective anti-black violence throughout 1919 are a complex entanglement of economic issues, political, and social problems. Soldiers in the millions were returning to their homes as they were discharged upon the end of the First World War and found themselves competing for jobs not only with civilians but also with other veterans. The fear of foreign ideologies (such as communism) and the labor movement caused tensions to flare. As the First World War shut out European immigration, another migration took place within the United States: The Great Migration, in which African Americans from the south moved in large numbers to cities in the north, completely changing demographics. These are all factors that historians have considered and looked deeper at, but conclusions often boil down to one, crucial point:
The primary reason for the racial violence that would become known as the Red Summer has everything to do with a desire by whites to uphold and reinforce white supremacy.
Although the First World War has gone down in American memory as an overlooked conflict, one that didn't really matter in comparison to its younger sibling that arrived two decades later, that feeling was not shared by Americans at the time. They knew it was a turning point in American history. Nowhere else was this thought prevalent than with African Americans. The specific American discourse surrounding democracy anchored in Woodrow Wilson's statement that the United States in the war would "make the world safe for democracy" sounded like an amazing opportunity for a people who continued to have their civil rights trampled upon throughout the country. If the United States was going to fight for democracy abroad, then they should also bring democracy to the United States. For their support of the United States in the war, the African American community expected that support to be reciprocated. The symbol of the African American soldier became particularly tied to this ideal. African American men, despite being heavily discriminated and abused throughout their service, went abroad and fought for the United States in France. They expected their service to be rewarded. At the very least, the restoration of civil rights and the destruction of Jim Crow was what they deserved for their service. We all know it turned out. Born out of the disappointed with the experiences during and after the war was a new type of racial attitude, a "New Negro", that would no longer accept the status quo. They wanted change and they wanted the full restoration of their civil rights.
The political and racial consciousness that grew out of the First World War amongst African Americans was seen as a threat by white racists for whom the racial hierarchy was a central part of their social and political world. Racial tensions were first and foremost "solved" through violence against black people, whether "rough justice" in the form of lynching or the larger anti-black violence that broke out 1917 in Houston, Texas, and East St. Louis, Missouri. African Americans had to be put back "in their place", they argued, and stay there. They should stay in their own neighborhoods, they should not aspire to take the white man's job, they shouldn't even think about earning more money than the white man.
This, therefore, became the primary reason for the outbreak of collective anti-black violence in Charleston, Longview, Bisbee, Washington D.C., Chicago, Knoxville, Omaha, Phillips County (Arkansas), Gary, and Bogalusa. While they were triggered by different causes, including anti-black propaganda that painted them out as criminals, strikebreakers, or simply unwanted -- the violence that followed was always initiated by white men and had no other goal other than to reaffirm white supremacy. Yet African Americans, the "New Negroes", would not go down without a fight. Since law enforcement and federal troops were unable and often unwilling to protect them, black men and women armed themselves. African American veterans put on their uniforms and patrolled in their neighborhoods. When attacked by white mobs, they resisted and defended themselves and lived up to the promise of no longer allowing white supremacists to get the last word.
Unfortunately, it never was this easy. White press coverage quickly blamed the violence exclusively on African Americans, sometimes tied in with contemporary anxieties around bolsheviks (with the implication that it was foreign demagogues who had riled up African Americans to go on a violent spree). Conspiracy theories of black uprisings meant to eradicate white people spread like wildfire. The majority of arrests and legal convictions during these violent outbreaks were black men. The majority of men whose weapons were confiscated were black men. The majority of people killed were black people. The fight for civil rights would continue, but the First World War was a turning point in the determination of getting America to pay their dues to those who were looking for democracy.
Sources:
1919, the Year of Racial Violence: How African Americans Fought Back by David F. Krugler (2015, Cambridge University Press).
Torchbearers of Democracy: African American Soldiers in the World War I Era by Chad. L. Williams (2010, The University of North Carolina Press).