“The Department of Homeland Security, like all executive agencies, must accomplish its mission while operating with finite resources,” Justice Department attorneys wrote in a court filing. “In fulfilling its charge to enforce U.S. immigration laws, it cannot arrest, detain, and remove all noncitizens unlawfully present in the United States. Thus, during every presidential administration, DHS invariably has had to make decisions over where it will dedicate its limited resources in order to further its objectives of national security, border security, and public safety.”