That is:
1) Not how international law actually works,
2) Not how anyone actually behaved in World War II,
3) Not remotely applicable before an official surrender is issued.
The law says that you have to call off military action and start releasing POWs into civilian life once their government surrenders. There is absolutely no legal or moral requirement to "let up" or "go easy" on a government that is strategically losing but refuses to surrender. This is especially so when the enemy has repeatedly invoked conditions of total war by violating the typical soldier/civilian distinction (eg: storing weapons in civilian dwellings, sending soldiers to attack without uniform or serial numbers, and various other tactics commonly labeled "terrorism" for lack of a unified legal category).
