St. Croix is an island in the Caribbean that is part of the United States Virgin Islands (USVI), an unincorporated territory of the United States. While it is geographically located in the Lesser Antilles, it has been under U.S. sovereignty since 1917, when the United States purchased the islands from Denmark for $25 million in gold to secure a strategic position in the Caribbean during World War I. Because it is a U.S. territory, residents of St. Croix are U.S. citizens, although they do not have a vote in presidential elections and are represented by a non-voting delegate in the House of Representatives. For travelers from the U.S. mainland, visiting St. Croix is considered domestic travel, meaning no passport is required for U.S. citizens, though a government-issued photo ID is still necessary. The island is the largest of the USVI and is known for its unique blend of Danish colonial history, vibrant Caribbean culture, and the stunning Buck Island Reef National Monument.