It is commemorating one of the survivors of the siege of Port Arthur, particularly the Shiro-Tasukitai (白襷隊). The survivor being commemorated is Yamada Rō, born in 1878 as Mike mentioned. It is not WW2-related, but it could have been presented (coincidentally) during WW2. There is no presentation date on it.
There is an organization called the Obon Society, or something like that. They try to repatriate flags such as this to Japan. If you search for it, you should find it (here on this site, or elsewhere).