P was sentenced to death for murdering a sheriff. P filed an application for a writ of habeas corpus in Federal District Court, challenging both his conviction and his sentence. The District Court conditionally granted the writ as to the sentence, mandating that P either be released or resentenced. The state trial court conducted a new sentencing hearing and again sentenced P to death. P filed an application for a writ of habeas corpus in federal court challenging this new sentence. The District Court once again conditionally granted the writ, finding constitutional defects in the new sentence. The Court of Appeals for the Eleventh Circuit reversed, holding P's challenge to his new death sentence was an unreviewable “second or successive” challenge under 28 U.S.C. § 2244(b) because he could have mounted the same challenge to his original death sentence. The Supreme Court granted certiorari.