Authentic flag with authentic writing...but I don't think it is from the war. It says,
けふよりは かへりみなくて 大君乃 しこの御盾と い出たつ吾は
which is an old poem that means: From today I/we humbly devote myself to your majesty's protection
The name after that is Takafuji Hanako (or could be read Takatō or Takadō, multiple readings are possible). She is presumably the person who wrote that poem on the flag.
The flag would presumably be addressed to Ōta Tetsuzō (the name going vertically down the left side).
The two biggest characters on either side of the red disc say 玉砕 or 玉碎, which would be a patriotic word/slogan imploring one to give up one's life for the country/emperor.
But the newness and cleanness of the flag, the lack of other people signing their names, the lack of honorific for the intended recipient, and overall "vagueness" of the flag make me think its a recently manufactured thing.