When you sue someone, you sometimes have a choice between filing in state court or federal court, and courts will generally defer to your preferred forum. In appropriate circumstances, however, a defendant can remove the case from state court to federal court. Under the current removal statute, 28 U.S.C. § 1441, removal is permitted by the defendant in any civil action brought in a state court of which the district courts of the United States have original jurisdiction. For those wishing to keep their cases in state court, care must be taken to ensure there are no grounds for federal-court jurisdiction. Some cases get removed to federal court before the plaintiff ever sees it coming.
The preemption doctrine can lead to such a result. Under this doctrine, a defendant may remove a cause of action that otherwise appears to lack federal question jurisdiction by asserting that federal law preempts the state law claim. This is because, under the Supremacy Clause of the Constitution, when state law and federal law conflict, federal law displaces (or preempts) state law.