sub base; minor base
A military base that is subordinate to or smaller than a main base. Used in contexts like the Japan Self-Defense Forces.
Air Self-Defense Force sub bases are scattered across the country.
基地 is a general term for a base (military or otherwise), while 分屯基地 specifically refers to a sub base or detachment base.
Compound of 分屯 (bunton, 'detachment, separate encampment') and 基地 (kichi, 'base').