Complex sentence: The rioters were overwhelmingly working class men, primarily ethnic Irish, resentful because the draft spared wealthier men, who could afford to pay a $300 commutation fee to hire a substitute.
Split into simpler sentences: The rioters were overwhelmingly working-class men, primarily ethnic Irish. They resented that the draft spared wealthier men, who could afford to pay a $300 commutation fee to hire a substitute.