Everyone knew that if France and Germany went to war, it would last a long time. When Hitler conquered France in about a month, everyone was shocked. It was at that point that FDR went all unconstitutional (IMHO) and arranged for military mobilization of American industry. And FDR also shipped as much military materiel to Britain as he could scrounge up.
Britain for its part, in addition to its prosecution of the war, shipped a fork truck pallet of top secret military technology to the US. Including the blueprints for the Merlin engine. And microwave radar.
The point is that American production of war materiel gradually ramped up between the Fall of France in May 1940 and Pearl Harbor. The first thing that entailed was building the buildings and making the machine tools required for the military production. The reason the US had a sparse inventory of military materiel was simply that FDR was shipping the stuff over to Britain - and later to the USSR - as fast as it was produced.
Hitler knew that the US was shipping war materiel to Britain, and so did his submariners. So even tho the Tripartite Pact was defensive in nature, and did not obligate him to declare war on the US after Japan attacked Pearl Harbor, Hitler was motivated to declare war on the US. Not doing so allowed too-easy passage of US aid to Britain and the USSR.
“Britain for its part, in addition to its prosecution of the war, shipped a fork truck pallet of top secret military technology to the US. Including the blueprints for the Merlin engine. And microwave radar.”
It was a lot more than that, and they still had some of the best nuclear physicists in the world at the time. I saw an interesting documentary about radar; we actually spent MORE money on RADAR than we spent on the A-Bomb, and the people who worked on it said that RADAR won the war; the A-Bomb just ended it. the Allies’ radar was so advanced that when the Germans captured some sets, they had a hard time understanding how they could have ever worked at all!
Thank you for the succinct overview!