San Juan, Puerto Rico
20th-century

Camp Las Casas, located in the district of Santurce, served as the main training camp for the Puerto Rican soldiers prior to World War I and World War II; the majority of the men trained in this facility were assigned to the "Porto Rico Regiment of Infantry" which was renamed the 65th Infantry Regiment of the United States Army by the Reorganization Act of June 4, 1920. The 65th Infantry was deactivated in 1956 and became the only unit ever to be transferred from an active Army component to the Puerto Rico National Guard.

Lieutenant Teófilo Marxuach (retired as a Lieutenant Colonel), a native of Arroyo, Puerto Rico, fired what is considered to be the first shot of World War I fired by the regular armed forces of the United States against any ship flying the colors of the Central Powers. Marxuach, who was a member of the "Porto Rico Regiment of Infantry" and Officer of the Day, on March 25, 1915, opened fire on the Odenwald, an armed German supply vessel, when it was trying to force its way out of San Juan's bay. The shots ordered by Lt. Marxuach were the first fired by the United States in World War I.

In 1919, Félix Rigau Carrera, "El Aguila de Sabana Grande" (The Eagle from Sabana Grande), the first Puerto Rican pilot, became the first native Puerto Rican to fly an aircraft in the island when he flew his Curtiss JN-4 from Las Casas. At the time, the area was used by the military as an air base and it was also Puerto Rico's first commercial airport, and Rigau Carrera was allowed to perform his historic flight from the airfield. Camp Las Casas was eventually closed down, and in 1950 a public housing project by the name of Residencial Fray Bartolome de Las Casas was constructed on its former location.

On January 2, 1947, the people of San Juan elected Felisa Rincón de Gautier (also known as Doña Fela) (1897–1994) as their mayor. Thus, she became the first woman to be elected as the mayor of a capital city in any of the Americas. During the Cold War era, she ordered the establishment of the island's first Civil Defense system under the directorship of Colonel Gilberto José Marxuach (Teófilo's son). Rincón de Gautier served as mayor until January 2, 1969.On October 30, 1950, San Juan was the scene of the San Juan Uprising, one of many uprisings which occurred in various towns and cities in Puerto Rico, by the Puerto Rican Nationalist Party against the governments of Puerto Rico and the United States. Among the uprising's main objective was to attack La Fortaleza and the United States Federal Court House Building in Old San Juan. The "La Fortaleza battle", which ensued between the nationalists and the police lasted 15 minutes and ended when four of the five attackers were killed.
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