Federal Government's Actions:
When a state doesn't comply with federal law, the federal government can take the following actions:
Suing the State: The Justice Department can file lawsuits against the state to compel compliance with federal laws.
Withholding Funds: The federal government can threaten to withhold funds that would otherwise be allocated to the state if the state doesn't comply with federal regulations, particularly if those regulations are related to the funding.
Direct Enforcement: The federal government can utilize its own law enforcement agencies, such as the FBI or Department of Homeland Security, to directly enforce federal law within the state, even if it doesn't have the cooperation of state authorities.
Judicial Review: If a state law conflicts with federal law, a party can sue in federal court, and the court can rule on the constitutionality of the state law, which will likely be struck down if it violates the Supremacy Clause.