The Battle of Stalingrad was the most important battle of WWII.

If men were more important than tanks, Italy would have lost the war with Ethiopia. Italy had three hundred thousand fewer soldiers than Ethiopia, but while it had over seven hundred tanks, Ethiopia had three. Tanks, planes and artillery guns are more important than men in wars. Men are relatively interchangeable. The population of the area kown as 'Grossdeutschland' was nearly a hundred million, the population of the occupied territories three times that. There was an almost unlimited supply of people to fight on all three sides in the war. The deciding factors were how well-equipped, organised, supported and financially backed those soldiers were.Organisation was something the Axis were pretty good at for the first stages of the war. The Blitzkrieg and Kampfgruppe doctrines were without parallel for the first three years of the war, and devastated the Polish, Dutch, Belgian, French, British and Russian armies incredibly well until it slowed.The other three factors were as a result of industry. From 1942 onwards, when the bombing campaign on Germany really started, Germany's industry did not cope well. As you said, the lack of anti-aircraft defences in Germany itself was partly due to the production of tanks in their stead. This also applies for consumer goods, tractors and other agricultural machinery, tools for improving infrastructure, military supplies, aeroplanes and trains. Without these, Germany lost its means for fighting war. The size of an army may have determined the victor in Napoleonic times, but in WWII, it was how well-supported the army was.Conversely, Russian industry was concentrated further east at places such as Yekaterinburg, Novosibirsk and Chelyabinsk, out of range of the German bombers. Allied industry was either in the fortress of Britain, which had had its anti-air defences renovated during and after the Battle of Britain and the Blitz; or in the USA, which was out of range of Germany and Japan.Germany could have still won the war if Kursk was won. The aim of Kursk was to encircle and annihiliate an entire Soviet Army Corps, totalling nearly two million men, twenty-five thousand artillery guns and five thousand tanks. Not only would the loss of a significant portion of the Red Army's men and materiel have caused a major shock to Soviet industry, meaning that they would have to transfer resources and manpower from other fronts to replenish the Ukrainian front. This would have resulted in a German surge that in all likelihood would have captured Leningrad, Moscow and the Caucasus oilfields.Also, the army corps at Kursk contained the modernised Red Army. It was a new model army that the Red Army was modeled on until the late 1960s, it was extremely valuable, and if it was annihilated or even defeated, it would deprive the Red Army of a means to fight the Germansin any organised fashion.Instead, the German army was almost completely annihilated and sent into full retreat. After Kursk, every German operation in the east became about slowing the Soviet advance while the front tapered and they could mount some sort of defense near the eastern border of Germany.