Why did most African Americans support U.S. participation in World War I? Was the support justified? What factors contributed to race riots and violence in the World War I era?

At the time of the First World War, the United States was a greatly segregated society. Many African Americans saw participation in the war effort as an opportunity to show their patriotism and perhaps improve their social standing and legal rights in the country. In fact, African Americans enlisted in such great numbers as soon as the country entered the war that the military stopped accepting African American volunteers within a week as quotas had already been filled.
Despite an eagerness on the part of many African Americans to serve in the armed forces, widespread institutional discrimination continued. Many positions in the military were not open to African American servicemen, and they served almost exclusively in segregated units.
This period also coincided with a migration of many African Americans from southern states to cities in the North, especially Chicago, New York, and East St. Louis. They were fleeing Jim Crow laws and lynchings and hoping for better job prospects in the urban North. They often found themselves competing with recently arrived groups of European immigrants for jobs and housing. Most riots were intended to keep African Americans out of these jobs and neighborhoods. Ironically, the military service of many African Americans only added fuel to the hatred that sparked these riots as some whites saw their service as their attempt to disrupt their racial standing in society.
In the end, despite the initial eagerness of many African Americans to enlist in World War I, they were still met with discrimination at home. Many people discounted or ignored their contributions to the war effort. The migrations to the North only put them in further conflict, which led to brutal race riots during and after the war.
https://armyhistory.org/fighting-for-respect-african-american-soldiers-in-wwi/

African Americans saw an opportunity in World War I to prove their patriotism, first and foremost, and with that hopefully gain status in the eyes of society back home after having served their country. The machine of war generated economic opportunities for African Americans as well because there were new jobs, and jobs vacated by white men leaving for duty, that those African Americans who were left behind were able to fill.
The support was justifiable in the eyes of many African Americans, at least at first, but was it justified? Idealism surely gave way to the realism of deep-rooted racism. Segregation was the norm not only back home but in the military as well. When African American soldiers returned from the war, they encountered hatred by whites who had seen workplaces include African Americans. Those same whites were often reluctant to see African Americans move into their neighborhoods. In short, the racism that existed before the war continued afterward unabated.
https://www.military.com/history/fighting-for-respect-african-american-soldiers-wwi.html