[amanaplanacanal]

From what I can tell, all Sanctuary City means is that locals will not cooperate with federal law enforcement unless it is legally required. Which seems right to me? States are independent entities with their own laws.

[dragonwriter]

Exactly, sanctuary city/state laws are an application of 10th Amendment reserved powers of the states, and particularly the principle known as the “anti-commandeering doctrine”, hinted at in in dicta concerning hypotheticals regarding the Fugitive Slave Laws in cases shortly before the Civil War and first applied as a basis for judgement by the Supreme Court in New York v. United States, 505 U.S. 144 (1992).

Even where the Constitution grants the federal government authority to make laws and to provide for their enforcement, it generally does not have the power to direct states to use their resources to enforce those laws. Sanctuary laws simply restrict the conditions in which state or local resources will be used to enforce certain federal laws.