The years after the turn of the century were very productive in the design and building of naval guns and gunnery. By 1915, several foreign battleships could outrange any harbor defense weapon in the United States. Moreover, the high firing angles of naval guns generally nullified the advantages of the disappearing carriages then used by the United States. During the war, many guns were removed from the existing seacoast defenses and remounted for use overseas. Many coast artillery units manned these and other field pieces in Europe. At home the coast artillery posts served as enlistment and training centers for those going overseas.
The U.S. Army entered a period of austerity following the end
of the war in 1918. Many of the coast defense forts were put on
caretaker status, maintained by a small number of soldiers, and
used as summer train-ing camps for reserve, national guard, reserve
officers training corps (ROTC), and civilian military training
corps (CMTC) units. New long-range 12-inch and 16-inch army ordnance
for sea-coast armament was built, supplemented by 16-inch naval
guns made available as the result of naval reductions due to the
Washington Naval Treaty of 1922. A number of new harbor defense
construction plans were drawn up, but few new batteries were actually
built during the this period.
A post World War I era battery-the gun platform of Battery Hall, Fort Saulsbury, DE
Mobile guns left over from the war were utilized as supplemental fire-power to support the older guns. 12-inch mortars and 8-inch guns and a few 14-inch guns on railroad carriages were sta-tioned at seacoast defense sites. During WW I, the United States had purchased from France a number of 155 mm GPF tractor drawn guns, which were later made in the U.S. as the M1918. In the late 1920s, these were pressed into a coastal defense role. To increase the accuracy of these guns, they were mounted on easily built circular concrete "Panama" mounts, which were named after the area in which they were first used. The growing importance of aircraft as an offensive weapon resulted in the formation and training of specialized Coast Artillery Corps antiaircraft artillery units during this period. A number of antiaircraft guns were installed at all harbor defense reservations during WWI and years thereafter.