This is not a Japanese item, so whether it is a flag or not is irrelevant. In Japan, tanks are called "sensha" (戦車), not "tank."
The hiragana spelling of the Japanese word "ike" (行け, meaning "Go!") is incorrect, so it was not written by a native Japanese person. Koreans and Taiwanese people of that era also received Japanese-language education, so it is unlikely that they wrote it either.
Japanese characters are normally written with a brush in this context. This example appears to have been written with a pen, and the writer seems to have traced over the letters multiple times.