Looks an interesting good-luck flag which was made before the writing style was formalized with names of the soldier, his family and friends.
It says -
トーチカ 皇軍 の 威力 に 降伏 (Tochika kogun no iryoku ni kofuku)
(Enemy's) Pillbox surrendered to our imperial army's power.
A scene from a cartoon postcard during the 2nd Shino-Japanese War circa 1937.