Walker (plaintiff) filed a products liability suit against Armco Steel Corp. (Armco) (defendant) in Oklahoma federal district court on the basis of diversity of citizenship. The complaint was filed before the Oklahoma statute of limitations had run on the claim, but service of process was not actually made until after the statute of limitations had run. An Oklahoma state law did not deem an action to commence for purposes of the statute of limitations until process was served on the defendant. Based on the running of the statute of limitations without commencement of an action, therefore, Armco filed a motion to dismiss. Walker admitted that his claim would be dismissed under the state law, but argued that Rule 3 of the Federal Rules of Civil Procedure governed when an action is commenced and thus preempted the Oklahoma state law. The United States Supreme Court granted certiorari.