It was the regular army component of the Harbor Defenses of the Delaware in World War II.

The 21st CA (Harbor Defense) (HD) Regiment was active from February 1940 until broken up in October 1944 as part of an Army-wide reorganization.

at Fort Pickens, Florida, but demobilized in December 1918 before organization was completed.

[2][3] Constituted in the Regular Army January 1940 as the 21st Coast Artillery (Harbor Defense) (HD).

Regimental HHB activated 1 February 1940 at Fort DuPont, Delaware by reorganizing caretaker Battery E, 7th Coast Artillery as HHB, 21st Coast Artillery.