Hard to say. The red seal says 朝鮮神宮 (Chōsen Jingū), which was a shintō shrine built in Seoul during the era of Japanese occupation of Korea. (The shrine was dismantled after the war).
The signatures and slogans are typical: "certain victory", "good luck in battle", etc... The addressee appears to be Shimizu Toshio, whose name appears prominently on the right. Next to that is a line that says "Older brother, Fight with Japanese spirit!", which is something slightly unexpected. The other signatures...there is a similarity to a lot of them, but I couldn't say with any confidence whether this is an authentic relic, or something doctored up after the war. It doesn't look like an obvious fake.