On Sept. 3, 1939, Britain declared war on Nazi Germany. On Dec. 11, 1941, Germany declared war on the U.S. On Aug. 8, 1945, Russia declared war on Japan. The point here is that for the best part of a thousand years, a convention prevailed that before one state waged war against another, it formally announced its intention to do so.
Belligerents’ diplomats were permitted to return unimpeded to their respective homelands — even the wartime Japanese and Germans went along with this, although they had launched surprise attacks such as that on the Day of Infamy.
