In all seriousness, Battles such as Stalingrad, The Battle of Britain and even Midway ARE important battles. You say that wars are decided by whole nations, i agree. But what if the Russian/Soviets had lost the battle of Stalingrad?
Germany would have gained a strategically insignificant city on the Volga and would have still been producing half as many tanks as Russia and of poorer quality, and would have been pushed back and lost the war no matter what. Stalingrad, though important part of the war, was not a battle that defined the outcome of the war nevermind human history. Even Tannenburg did in the first world war, or Tsushima did in the Russo-Japanese wars, did more to define the outcome of the conflict, and therefor, to a greater extent, human history.
Even in WWII, there are more strategically vital battles than Stalingrad or Midway, but have left a much smaller hagiography behind them. Surely you would think it odd to include the Battle of Tunisia, or the Marianas Islands, as the most significant battles in history, but these were even more decisive then Stalingrad or Midway.
What I'm surprised to see is that all these battles are considered important by the winners of the war, but this is something of a mistake. If I had to pick the most important battle of WWII, it would be the Fall of Singapore, because it heralded the end of colonialism world wide.
I'm not going to attempt a list of all of Human History, because my focus of study is to narrow. I will put out the list of the most influential battles of the twentieth century, as I see it.
1.The Battle of Tannenburg- The very image of a decisive victory. Prevented an early defeat of Germany in 1914, that could have resulted in the war ending by 1915 or 16. But most importantly it Brought about the collapse of the much feared Russian Army, and ultimately led to the single defining event of the Twentieth Century, the creation of the Soviet Union.
2. The Battle of Tsushima Straights - I feel rather sorry for the Russians, putting the two major battles of the century as stunning defeats for them. But these were the battles that achieved decisive change. Tsushima of course, was the first time a non-white nation had defeated a white nation in a major conflict since the Turks threatened Europe. Additionally, without this battle, Japan would have lost the war. Had Russia declared control of the sea lost, and limited the conflict to Manchuria and the Russian Far East, they would have soon won. Japan was going bankrupt and could not reinforce its troops, while trains carrying Russian troops were finally making their way east.
However the decision to deploy the Baltic Fleet to Asia resulted in the decisive victory the Japanese needed to sue for peace.
This had numerous outcomes important for the twentieth century. First, while it did not herald the fall of colonialism the way Singapore did, it did show that Europe no longer maintained a strategic monopoly on power, and that not only was Japan to be considered a major power, but that Asians could assume power, and therefor a re-estimation of China's strategic value.
Second, it
did catch the attention of nationalists around Asia. However in a different way then Singapore later would. Tsushima made sure that the Meiji restoration: rejuvination of sciences, abandoning of traditional confines, and crash industrialization, would serve as a model for nationalist movements.
Third, it had a major impact on the domestic front for Tsar Nicholas II, and led to the unrests of 1905, and the beginnings of many revolutionary movements.
Lastly, and perhaps most importantly,it had a fundamental effect on the outlook of the IJA and IJN. For them, Tsushima reinforced the point they had taken from the first Sino-Japanese War and made it explicitly clear to them. They had won the war, but had victory taken from them at the peace table. Japan gained no foothold in Manchuria, was denied Port Arthur, and Vladivostok did not become demilitarized. For many, this would be the defining turn from the Prussian style nationalism of the Meiji Era, to the virulent Ultranationalism that would brook no compromise of the Showa Era, the implications of which do not need to be outlined.
3. Battle of Verdun I joke sometimes that the Battle of Verdun was the battle that made France loose the
second world war, but its true. The fighting at Verdun, though not particularly more brutal than at say, the Somme was important for two main reasons. First of all, there was the cycling of troops. This was an important way to keep morale high, during the long battle, but it also created a fundamental change in the way war impacted the populace. For the first time in war, the vast majority of troops engaged in some of the worst fighting. There were virtually no isolated troops serving on quiet fronts, as troops began circulating in and out of Verdun. So many troops being involved in the fighting had an immense role in shaping postwar attitudes about war and a future conflict.
Second, it made Phillipe Petain a heroic figure in French politics, and ensured that he would have suitable clout to form a government later. If you want to understand the collapse of France in the Spring of 1940, you must begin in 1916.
4. The Fall of Singapore
The fall of Singapore, as afforementioned, heralded the end of the British Empire. At her strongest point, at the place she claimed to be invulnerable, she was defeated by an outnumbered Asian power, only shortly after Japanese Naval Aircraft sunk Force Z. For the first time the imperialists were layed low, and though the Japanese were not truly fighting for Asian freedom, the implication was clear to leaders around Asia: the Europeans are very mortal, and their time has passed.
5. The Attack on Pearl Harbor
This battle (and it was a Battle) defined how WWII would be fought. Pearl Harbor ensured that for Germany and Japan would only be allowed absolute, unconditional surrender (the decision to apply the same to Italy would come much, much later, at the rather bone-headed insistence of Anthony Eden). Had the Japanese merely attacked the Phillipines, or Malaysia, there may have been some room for a brokered peace with them. However the Surprise attack on Pearl Harbor had such an impact on the American Psyche, that unconditional surrender was the only outcome possible, heralding the nuclear age.