The Guard Districts (警備府, Keibifu) were second tier naval bases, similar to the first tier Naval Districts (鎮守府), with docking, fueling and resupply facilities, but typically lacked a shipyard or training school.
[1] They tended to be established by strategic waterways or major port cities for defensive purposes.
the Guard District maintained a small garrison force of ships and Naval Land Forces which reported directly to the Guard District commander, and hosted detachments of the numbered fleets on a temporary assignment basis.
Mako was designated a third echelon naval port, or yokobu (要港部) on 4 July 1901.
Mako was upgraded to full Guard District status on 20 November 1941, and served as a staging group and supply base for the invasion of the Philippines and other naval operations in Southeast Asia after the start of the Pacific War.