From the Document: "Jurisdictions with 'sanctuary' policies have been controversial for decades, highlighting a tension between federal agencies charged with immigration enforcement, and states and localities (e.g., counties, cities) that limit their cooperation in this regard. The prominence of immigration enforcement for the Trump Administration, and the publicity surrounding certain crimes committed by some non-U.S. nationals (in statute, 'aliens'), particularly those without legal immigration status, have reignited debates over appropriate levels of immigration enforcement within the United States. In 2017, President Trump issued Executive Order 13768 (EO), which included provisions that aim to deter certain 'sanctuary' policies and promote cooperation with federal immigration enforcement agencies."
CRS In Focus, IF11438
Congressional Research Service: https://crsreports.congress.gov/