Re: Japan, there were several reasons it took the US several years to finish them off. For starters, Germany First meant that only a limited portion of US industrial capacity was devoted to the defeat of Japan. If all of America's warmaking potential had been directed towards the Pacific then the US could have advanced much more rapidly.

Second, the logistical difficulties of moving armies across the vastness of the Pacific Ocean are immense. I've recently been working my way through John Lundstrom's Black Shoe Carrier Admiral, and it's extremely interesting to see how badly the US Navy was constrained by ability to supply fuel to its ships in the early part of the war. In 1942 just keeping a couple of carrier task forces adequately supplied with fuel oil stretched the American tanker fleet to its limits. Defeating Japan didn't just mean building a lot of warships, it also meant developing the infrastructure to keep those ships fueled, supplied and repaired (ships break down a lot) thousands of miles away from their home ports and shipyards.

In addition to building huge amounts of stuff, the US Navy had to develop a great deal of institutional knowledge in order to overcome their logistical challenges. At the beginning of WW2 they were barely capable of refueling warships at sea. I think it's remarkable that they were able to develop the skillset they needed and train the vast numbers of personnel needed for the vastly expanded fleet that was build over the course of just a few years.

One could argue Germany’s persistence was a result of the allies bumbling.

The allies split a lot of resources eg. The bombing campaign- Germany’s production increased until then end of the war, which is incredible considering the strategic situation, which indicates that the bombing campaign was not effective. The main problem was the allies failing to target German transportation and electrical networks until rather late and focussing too much on bombing cities.

Anthony beevor also made the argument in the Pacific theatre where Nimitz intended to starve Japanese forces on islands with no attacks necessary. However he had to contend with MacArthur who had famously stated he would return to the Philippines. Thus the navy’s plan was frustrated and the island hopping was brought about.

This leads to a further point of the clash of personalities and infiltration of politics into the allied forces due to the belief in Germany and japans inevitable defeat. You have the drawing up of Europe between the western allies and soviets before the war was over. The political struggle between socialists and de Gaulle during france’s liberation.

After the battle of the bulge Montgomery’s claims to have saved the day nearly provoked a diplomatic incident. The fact such a thing could come from a comment indicates that the allies had started to look beyond the war as no longer so urgent. The situation was essentially under control by that point. The allies didn’t even think something like the Ardennes offensive was possible, further pointing to a belief that the war was essentially over.

There was also the various operations that either failed or were poorly thought out. The invasion of Norway stands out as a waste of resources. Market garden could have been successful but it was complicated, suffered from some poor equipment and divisions didn’t push hard enough so to speak or at least were not prepared to.

Further to this the Allies missed many opportunities to inflict greater damage on Germany. I already mentioned the poorly targeted bombing campaign but there was also the failure to fully close the falaise pocket, allowing many German officers to escape to join reconstituted army units. The failure to capitalise on the Germans failure in the Ardennes offensive. The failure to prevent the axis forces escaping Sicily.

The allies proved themselves no push overs fighting the Germans as the course of the war post d-day attests. The bulge offensive can said to be doomed from the start considering the opening engagement.

In summary the allies inability to capitalise on their successes, efficiently use resources and seize the initiative lead to the prolonging of the war. Politics and internal rivalries also didn’t help. Strong personalities like MacArthur actually overrode perfectly good strategies for their own agendas.

The German army for all their fame had very little in the way of success following The failure to seize Moscow. From Stalingrad on the eastern fronts collapse had begun and once Africa was lost the collapse in Europe had begun. They owe their longevity during the war to the allies rather than their own ability. Not saying the Germans didn’t know what they were doing but they themselves were guilty of wasting resources (personal opinion being the desire to make ridiculously technical and large tanks) and they had to contend with the lost initiative, lost air superiority. Further to this the wasted potentially friendly populations eg, Ukrainians, German Jews etc. Due to the Nazi party’s doctrine.

I think it’s important to look past some of the myths of this army was better and why etc etc. The allies armies boasted many a career soldier. The Soviet Union was fighting the Germans on even terms once it had resolved the fact it had removed its own officer corps.

Also getting into the disproportionate casualties bear in mind the civilians in The USSR and Poland that had to bear the brunt of the German military’s brutality. In conflict with the western allies casualty rates definitely favour the allies.

I would highly recommend taking a look at the war in China if you want to see the depths of politicking and rival personalities in the allied camp.