[2] Admiralty Inlet was considered so strategic to the defense of Puget Sound in the 1890s that three forts—Fort Casey on Whidbey Island, Fort Flagler on Marrowstone Island, and Fort Worden at Port Townsend—were built with the intention to create a "triangle of fire" against invading ships.

This military strategy was based on the theory that the three fortresses would thwart any invasion attempt by sea.

[4] Designed as part of the massive modernization program of U.S. seacoast fortifications initiated by the Endicott Board, construction on Fort Casey began in 1897.

The invention of the airplane in 1903, and the subsequent development of military aircraft made the fort vulnerable to air attack.

Most of Fort Casey's guns and mortars were removed during World War I, when they were mounted on railcars to serve as mobile heavy artillery.