The first U.S. national historic site designated was the Casa Blanca in San Juan, Puerto Rico, recognized in 1949. This historic home, built in the 16th century, served as a residence for various governors of Puerto Rico and holds significant historical and cultural value. Its designation as a national historic site helped to preserve its architecture and heritage. As one of the oldest buildings in the Western Hemisphere, it provides insights into early colonial life and reflects the diverse history of Puerto Rico under Spanish rule.