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Obviously, the worst three presidents--by competence--were James Carter and Barack Obama.
Nobody else even came close.
Clinton was indolent, morally bankrupt, and lucked into a fairly peaceful and prosperous period engineered by Reagan and G. H. W. Bush. Had Clinton been faced with the Soviets, the invasion of Kuwait, or 9/11 ... well, I'm just glad he wasn't.
FDR, in mind, is a difficult case. On one hand he was a fairly effective leader during WW2; on the other he took Hoover's government-intervention-ism, dialed it up to
111, and made long-term the depression Hoover had made worse than it need have been. I seriously do not think the Great Depression would have ended before FDR left office, but for Hitler, Tojo, and Mussolini. Further, FDR's
gut-the-military method of funding his social programs - now sacred doctrine among Ds - cost thousands of good men's lives, To list a few examples:
* The USN's Mark 13, 14, and 15 torpedoes had just 3 flaws - they ran way deeper than set, the magnetic exploder could not work reliably, and the contact exploder was too fragile to work reliably; this could have been learned, had they been adequately tested and used in training; those were the torpedoes used by USN destroyers, submarines, and torpedo bombers; reliable torpedoes came into use in the second half of 1943, nearly 2 years after Pearl Harbor;
* USN and Army Air Corps fighters were half to a full generation behind the fighters the Japanese and Germans were flying; the P-39, P-40, and F4F were largely out-classed; the P-38 lightning, which was the peer of German and Japanese fighters entered the war nearly a year after PH, as did the P-47; the USN F6F Hellcat entered the Pacific Theater nearly 2 years after PH;
* USN was badly under-trained in surface combat, especially in night fighting, and
the IJN was insanely well trained in night fighting;
* Worse than fighter aircraft, US Army tanks on entering WW2 were a generation or more behind what they would face in Africa and Europe; the M3 Lee mounted its 75mm main gun low, in a sponson rather than high in the turret (which had a 37mm gun of dubious usefulness); the M4 Sherman came into use in late 1942 and was adequate against German tanks in the field, but quickly fell behind.
IOW, even though FDR was a good leader in WW2, his social program spending was funded by gutting the military. US Army, Navy, and Marines paid for those failed social programs in blood. I could easily persuade myself that FDR belongs on my
Worst Presidents list.