Short answer: The U.S. Supreme Court determined the rights of Puerto Ricans in the Insular Cases, a series of decisions issued in 1901 (with related cases through the early 20th century) after the Spanish–American War. These rulings established that residents of unincorporated territories like Puerto Rico were not automatically entitled to all constitutional rights, framing them as property of the United States but not fully incorporated within its constitutional protections. Context and key points

• Historical backdrop: After the Spanish–American War (1898), the United States acquired Puerto Rico, Guam, and the Philippines. Congress and the Supreme Court faced questions about whether and how the U.S. Constitution applied to these new territories and their inhabitants [Insular Cases overview and analysis].

• Core holding of the Insular Cases: The Supreme Court held that full constitutional protection does not automatically extend to unincorporated territories; only “fundamental” rights and those explicitly bound to incorporation would apply. This created a distinction between incorporated and unincorporated territories and allowed the U.S. to govern territories with limited constitutional constraints [Insular Cases overview].

• Citizenship developments in Puerto Rico: The broader jurisprudence around Puerto Rico’s status evolved over time, including statutory citizenship provided by Congress (e.g., Jones–Shafroth Act of 1917) but with ongoing debates about the scope of constitutional rights for Puerto Ricans. The 1917 act granted U.S. statutory citizenship, which is distinct from full constitutional citizenship, and subsequent cases further clarified the rights landscape in Balzac v. Porto Rico (1922) and related decisions [Jones–Shafroth Act context].

• Critiques and ongoing debate: The Insular Cases have faced substantial critique for their racial and colonial implications, and recent scholarship continues to reexamine their legitimacy and impact on rights in U.S. territories [Harvard Law School analysis; contemporary scholarship].

Notes for further study

• If you want a concise list of the specific cases commonly grouped under the Insular Cases (e.g., De Lima v. Bidwell, Armstrong v. U.S., Balzac v. Porto Rico, Downes v. Bidwell), I can outline the key holdings and which rights were considered fundamental vs. non-fundamental.
• For a contemporary legal perspective, there are ongoing discussions about overruling or reinterpreting these decisions in light of modern constitutional rights and territorial governance. This includes scholarly pieces and official summaries from legal research centers [Harvard Law School; U.S. Commission on Civil Rights materials].

If you’d like, I can pull a brief timeline of the Insular Cases and summarize the exact holdings of the major opinions relevant to Puerto Rico.