During the 1850s, Congress had resisted a demand for Arizona statehood because of a well-grounded fear that it would become a slave state.
The U.S. Civil War was occurring at the time, and the Union needed silver, which Trimble explains as being one of the main reasons for passage of the Act.
[1] The New Mexico Territory had a long history of enslavement of Native American people, first by each other and later by Hispanic settlers (cf.
Although in 1860 there were relatively few African American slaves in New Mexico, the legislature formally approved slavery shortly before the Civil War.
According to historian Martin Hardwick Hall, invading Confederate troops brought an unknown number of enslaved African Americans into the territory.