1) The Battle of Mukden-Arguably the battle that began de-colonization and cut one of the major tenets of White Supremacy down to size. One of the largest battles before the world wars, and a definitive victory for the Imperial Japanese Army. In terms of a global sense, definitely qualifies for number one.
2) Battle of Dien Bien Phu-Vo Nyguen Giap's victory over the French here arguably paved the way for all else that followed in the Indochina Wars, as well as establishing the rule that in colonial wars, try as people might, they weren't going to stop anti-colonial movements from securing independence by force of arms if they had to do so.
3) Operation Yellow-The invasion of France was arguably the moment that WWII went bad for democracy and communism alike, and it was perhaps the most decisive event of that war for the Axis side. It's the clearest example of a single decisive event *on* the Axis side.
4) Battle of Singapore-The battle that broke the British Empire. Also the one instance in WWII together with Malaya where the IJA actually fought at a level worthy of admiration.
5) Operation Bagration-The most decisive Soviet victory of the Second World War, annihilating Army Group Center and taking the USSR to the Vistula, and putting in place the key elements of the future Warsaw Pact.
6) Battle of the Ruhr Pocket-The greatest Allied victory of WWII in the West, with this the war in the West in terms of a serious battle was over. It was the greatest encirclement operation of Anglo-American warfare and is shamefully neglected like everything Western Front that's not D-Day.
7) Battle of the Bar Lev Line-Arguably one of the most decisive battles in 20th Century Middle Eastern history, as it was the moment Israeli invincibility choked on its own mythology. Also the greatest moment of achievement for Arab arms in the 20th Century. In this sense tied with the Third Battle of Gaza and the Battle of Megiddo as decisive battles in the region.
8) The Dardanelles Campaign-By itself nearly killed off amphibious warfare for good. That is no mean feat.
9) The Second Chinese Strategic Offensive-This one was a decisive victory for the PRC and helped establish its reputation and prestige in a real extent to this very day, such as it is, under arms. It was one of the few instances in modern war where the rock smashed laser to bug-dust. Douglas MacArthur may have helped make it so, but the PRC certainly had a moment of awesome here.
10) The Battle of Medina Ridge-The largest tank battle in modern history, and so lopsided a victory for the USA that it deserves mention in no small part for establishing the rule of the USA as the military 800 lb gorilla.
11) The Battle of Stalingrad-Arguably the point at which the Second World War changed from if the Allies would find the will and the ability to win, to how long it would take to crush Nazi Germany and who controlled what when, where, and how.
12) The Hundred Days Offensive-Earns last place because it was perhaps the most decisive victory in a real sense in either of the world wars, won by an alliance that felt it was on the outs a few months prior and by the end of the fighting here a few more weeks and the German army turns into an armed Mob.
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Which battles would you pick? Or do you think in modern warfare there can be a decisive battle at all?