In April 2010, the Arizona State Legislature passed and Governor Janice K. Brewer signed into law Arizona Senate Bill 1070 in an effort to deal with illegal immigration, escalating drug and human trafficking, and other public safety concerns. The new law, set to go into effect on Thursday, July 29, 2010, empowers Arizona's state and local police forces to enforce strict new provisions against illegal immigrants in the state. Among its various provisions, S.B. 1070 requires that state and local police officers in Arizona check a person's immigration status when enforcing other laws, and it further authorizes officers to make warrantless arrests of any individuals when probable cause exists to believe that the person has committed a public offense that would make that individual removable from the U.S.

On July 28, 2010, U.S. District Court Judge Susan R. Bolton issued a ruling in the case of United States v. State of Arizona et al, in which the federal government as plaintiff challenged the constitutionality of S.B. 1070 on the grounds that the power to regulate immigration rests solely in the federal government, and that the Arizona law is therefore preempted by federal law. In her ruling, Judge Bolton found that the U.S. is “likely to succeed on the merits in showing that” some sections of S.B. 1070 are preempted by federal law, and therefore issued a preliminary injunction on those sections.