During World War I the Army constructed numerous temporary buildings on the site of the 1915 Panama–Pacific International Exposition at the Presidio of San Francisco and linked it to Fort Mason with a rail spur. In July 1918 Congress passed Public Law 189 to establish eight "air coast defense stations" and appropriated $1.5 million for the construction of one of them at the Presidio, to protect San Francisco Bay. In June 1919 the Army assigned Colonel Henry H. Arnold of the Air Service as Air Officer, Western Department, and directed him to convene a board of four officers to select the site. The board chose the former exposition site as much for its sheltered beach to protect seaplane operations as the fact that the infield of its racetrack was already in use as an aviation field. Although the wartime appropriations were reduced by the end of the war, demolition of buildings posing a landing hazard began in the fall of 1919.