What court cases define when federal officers are immune from state prosecution for actions taken in the line of duty?

1. In re Neagle: the founding precedent that created Supremacy Clause immunity

In re Neagle (135 U.S. 1) stands as the foundational Supreme Court decision holding that a federal officer who uses force while discharging federal responsibilities may be immune from state criminal prosecution because federal law is “supreme” and can displace hostile state enforcement—Neagle’s arrest by California and subsequent federal habeas relief exemplified that doctrine ^[1].

2. Martin v. United States : the Court’s recent course correction

The Supreme Court in Martin clarified that Neagle’s reasoning historically protected federal officers from state criminal prosecution but does not automatically displace statutory regimes like the Federal Tort Claims Act; the Court emphasized that Neagle had been about shielding officers in criminal prosecutions and cautioned against treating that decision as a general exemption from federal statutory liability ^{[4] [1]}.

3. Supremacy Clause immunity in practice: lower-court standards and variation

Because the Supreme Court has provided limited modern elaboration, circuits have developed divergent tests—some require that an officer act within authority and reasonably believe the act was necessary to fulfill federal duties, while others apply more deferential approaches that often favor immunity; the Tenth Circuit’s “objectively reasonable and well-founded basis” formulation and the Ninth Circuit’s handling in Idaho v. Horiuchi illustrate this patchwork ^{[5] [6]}.

4. Qualified immunity: a different animal mostly for civil suits

Qualified-immunity precedents such as Harlow v. Fitzgerald and Anderson v. Creighton govern civil liability and protect federal officials from damages suits unless they violated “clearly established” law or reasonable officers would have known a constitutional violation occurred; those doctrines do not directly answer when state criminal prosecutions must be dismissed under the Supremacy Clause ^{[3] [7]}.

5. The FTCA, state tort claims, and the limits of Supremacy-based defenses

The Federal Tort Claims Act waives the government’s sovereign immunity for certain torts and creates a separate path from criminal prosecutions; the Supreme Court in Martin stressed that Supremacy Clause immunity traditionally functioned as a shield in criminal cases and that invoking that shield against FTCA claims raises distinct statutory and preemption questions ^{[1] [8]}.

6. The practical landscape: removal, federal adjudication, and sparse Supreme Court scaffolding

When federal officers face state charges they typically seek removal to federal court and assert Supremacy Clause immunity as a federal defense; scholars and commentators note that because the Supreme Court has rarely revisited Neagle, federal courts remain the primary arbiters—leading to inconsistent outcomes and the observation that the Court has “not been generous” with modern guidance ^{[9] [2]}.

7. Where doctrine is unsettled and what that means for accountability

Legal scholars and advocacy groups stress that the doctrines overlap uneasily—Supremacy Clause immunity for criminal prosecutions, qualified immunity for civil suits, and the FTCA for statutory claims—creating gaps in remedy and accountability; courts have sometimes denied immunity where evidence suggested deliberate malice or clear departure from authorized federal duties, but overall the terrain is fact-intensive and fragmented ^{[2] [9]}.