Your flag is oriented vertically in the picture, when it should be oriented horizontally.
The red stamp, large letters and the threads go on the right side. The large letters read (from top to bottom when oriented correctly) 祈武運長久 (pray for everlasting luck in battle).
The red stamp contains the name of a Shintō shrine located around the Ōsaka area, Kōzugū.
There are two other characters on that red stamp that I am unable to read. Stay tuned...
On the extreme far right (again, when oriented correctly) is the name of what looks to be some god/deity associated with Okinawan Shintō, Kimitezuri (君手摩), but again there are two other characters I can't make out. I don't know why the flag would have an Okinawan phrase as well as the name of a shrine in Ōsaka on it. (I could be wrong about the Okinawan god, so stay tuned on this as well...).
The rest are names of people who signed the flag.