Mandarin speaker here. The four characters read from right to left 『久長運武』roughly translates to "continued luck in the fortunes of War." I'm not entirely sure but I'm guessing rest of the kanji written on the flag are the names of the Japanese soldiers who signed the flag.

Edit: grammar

I'm not able to answer your question, but that is an incredible piece of history. Take good care of it, OP.

Maybe try /r/Japan ?