Military history of Puerto Rico – Wikipedia

Puerto Rico was considered the “Key to the Caribbean” by the Spanish because of its location as a way station and port for Spanish vessels. In 1540, with revenue from Mexican mines, the Spanish settlers began the construction of Fort San Felipe del Morro (“the promontory”) in San Juan. With the completion of the initial phase of the construction in 1589 El Morro became the island’s main …

https://en.wikipedia.org/wiki/Military_history_of_Puerto_Rico