The Humanitarian Law Project (HLP), five other organizations, and two individuals (collectively Plaintiffs) filed suit in district court against U.S. Attorney General Eric Holder, Jr., and others (collectively Defendants) challenging the constitutionality of a federal law that prohibited the giving of “material support or resources” to certain foreign organizations designated by the Secretary of State as engaging in terrorist activities. 18 U.S.C. § 2339B(a)(1). Specifically, Plaintiffs sought to provide training, education, and other resources to two designated terrorist groups: the Kurdistan Workers’ Party, also referred to as the Partiya Karkeran Kurdistan (PKK) and the Libertarian Tigers of Tamil Eelam (LTTE). After a long, protracted litigation effort during which Congress amended the statute, the district court partially granted Plaintiffs’ motion for a preliminary injunction. Defendants appealed. The court of appeals affirmed. The U.S. Supreme Court granted certiorari to review.