In 1863, what development caused four days of the worst rioting americans had yet seen?

During the Civil War in 1863, Irish immigrants, along with others caused riots lasting for four days in New York City, targeting African Americans. This was due  attempts to apply the Union draft by Congress which called for men, mostly working-class, of Irish descent to fight in the ongoing war. The rioters feared that the black people, if freed, would end up competing with them for jobs.