This flag was taken by the 6th Infantry Division, probably taken at Luzon, Philippines. The members of the US division also signed the flag. No other information available. I will be happy to provide additional photos. Please forgive me if the orientation is wrong.
It's not genuine since 1) 祝 is written in the direction upside down, and 2) 必勝 and the signature is written in the back side to the seal of a shrine (櫻坐神社?).
No. It's not the problem of the direction of your picture. The problem is on the written direction of the slogan and the written side of the slogan/signature themselves. Anyone can write anything on a blank genuine flag afterward. There are quite so many examples of those kinds of flags.
Here's the correct direction to the shrine seal. The slogan in the blue circle is written upside down, and the slogan and signature in the red circle are written on the back side.
It was obtains at an estate sale along with a book about the 6th infantry and photographs taken while the soldiers were in the Philippines. The names written on the flag were from the squad of the man that had the sale.