Hello Joe,
Horizontal orientation is correct. The big black characters across the top (right to left) say 祈武運長久 (pray for everlasting luck in battle). The big characters arranged vertically on the left say 為高木紘一君 (For Mr. Kōichi Takagi). The rest are the names of the people who signed the flag. It also has the signature of the mayor of a town, but I can't make out the town name 囗松町長山下清之助
(Mayor of Something-matsu, Yamashita Kiyonosuke) The character in red is the one I can't figure out. The character in orange I'm not 100% sure about either.