The US was under-militarized at the beginning of the WW2 years, but its industrial capacity was unrivaled and undeniable. Even before its entry into the war, for instance, it was almost singlehandedly supplying the British and Soviet war efforts. The US was supplying nearly 40% of all the USSR's food, clothing, munitions, and military hardware during Hitler's Barbarossa campaign, in the months prior to Pearl Harbor. It was doing all of this while its economy was roaring back up from the Depression, and its output was just getting started. (The US produced about 2,500 warplanes in 1940; by 1944, it was producing just under 100,000 per year.)
Germany and Japan correctly feared the United States long before its entry into the conflict. They feared it not necessarily as a war machine, but as a relentless and massive industrial machine. They knew that, if war with the US were to drag on for a few years, the US would eventually get its shit together and outproduce and outgun either of them by an order of magnitude.