Whether the federal circuit court had federal-question jurisdiction over the Mottleys' suit.
Holding
No. The suit did not arise under the Constitution or laws of the United States because the Mottleys' own cause of action was a state-law contract claim.
Reasoning
Federal courts are courts of limited jurisdiction, and the Supreme Court must examine the lower federal court's jurisdiction even when neither party challenges it. Here, there was no diversity of citizenship, so jurisdiction could rest only on the proposition that the suit arose under federal law.
A suit arises under federal law only when the plaintiff's properly pleaded statement of its own cause of action establishes that the claim is based on the Constitution or federal law. The Mottleys' affirmative claim was for enforcement of a contract to provide transportation; that claim did not itself depend on federal law.
The Mottleys invoked federal law only by anticipating the railroad's expected defense: that the 1906 Act prohibited the free passes. They then anticipated their own response, arguing that the Act did not apply to their contract or would be unconstitutional if it did. An anticipated federal defense, and an anticipated reply to that defense, cannot create federal-question jurisdiction.
The Court relied on prior cases applying this pleading rule, including Tennessee v. Union & Planters' Bank and Boston & Montana Consolidated Copper & Silver Mining Co. v. Montana Ore Purchasing Co. Those decisions establish that a plaintiff may not obtain federal jurisdiction by pleading a likely defense before the defendant raises it.