[hibikir]

Did you ever read on the early parts of the war, during the race to the sea? Both sides were extremely low on ammunition, as they realized that their ideas of how much ordinance would be used in a real war were completely wrong. It took a really long time to ramp up production capacity to get anywhere near Verdun, and for the biggest horrors of WWI's trench warfare to appear.

Everyone kind of learned their lesson for WWII, as the real signal that war was coming wasn't actual invasion, but the ramping up of industry to actually supply combat. The US intervention in Europe couldn't have happened much earlier, because setting up the logistics to do any intervention whatsoever took years... and this was at a time where, while the US still claimed neutrality, the country was already changing their industrial production to be able to begin intervention.

So even in the good old days, nobody could change their industrial capacity to supply a war on a dime. And if there is no war, the costs of dedicating so much production to unused equipment is prohibitive