Certainly not to disagree with you. Just to point out that where Declarations of War had previously been considered diplomatic necessities, by 1939 it was apparently strictly optional, and often not so desireable, and sometimes didn't even mean real war -- as your example of the French demonstrates.
Even Roosevelt in 1941 did not ask for a straightforward "Declaration of War" on Japan, but instead the rather curious turn of phrase he asked for was: Congress to declare "that since the...attack... a state of war has existed."
I conclude, the declarations themselves, when they even happened were meaningless, and that may explain why we've seen none since?
OK, I get it now (you can visualize me smacking myself on the head).
Back in the day a “Declaration of War” had meaning. Up through the Napoleonic Wars it took months to get the armies in position to fight. Up to World War I, the doctrine of mobilization meant it would take weeks to get the armed forces ready to fight.
By the time of World War II, with modern transport and communications equipment, and the fact that so many countries were democracies with deliberative forms of government, the idea that you would “debate a declaration of war” just invited a quick attack on your forces before they were ready. The lesson of World War II, carried through the Cold War, was that you were always ready because in the era of ICBM’s equipped with thermonuclear weapons, war was going to come very quickly and without warning.
I guess the “Declaration of War” has become obsolete by the pace in which we now live. Any modern country that has to go through the motions of “Declaring War” doesn’t really have the stomach for one. France 1939 invented the axiom.