It is difficult to pinpoint one war and say it is the most "important" war in human history. There have been many conflicts which have greatly altered the ripples of history, sometimes greatly affecting the world for hundreds or thousands of years (Greco-Persian War, the Punic Wars, the Battle of Kosovo, the Manchu Wars in China, the Napoleonic Wars, the United States Revolutionary War, the Russian Civil War, etc etc etc). For the sake of argument, I will give my examples of one of the major conflicts which abruptly turned the world on its heel and altered history indefinitely. I feel, that based on the historical context, these conflicts were the "most important" wars/battles.
The Roman defeat at Teutoberg Forest (http://en.wikipedia.org/wiki/Battle_of_the_Teutoburg_Forest)
I feel that this had the greatest impact on Western civilization. By not only losing, but losing embarassingly, at the ambush at Teutoberg, Publius Quinctilius Varus signed the death note to the Roman Empire. It's all a very slippery slope, but I feel that it is a very safe argument to make. He doomed the Roman Empire and greatly altered history.
By being lured and trapped into the fight, Varus and his legions were completely surrounded and vulnerable to a much smaller, clumsier, and less armed German army of brigands. The three legions were the proverbial "sitting ducks" and were slaughtered in due form. This defeat was overwhelming, and not even based on numbers alone, with the Romans losing between 15 and 20 thousand soldiers.
They key to this defeat is morale. The defeat gave the Germans a newfound sense of pride. They had defeated, handily, the great and powerful forces of civilization! Hermann, the German leader (who had beguiled Varus into the trap, as Hermann [Arminius] was a former Roman hostage who had become Varus' advisor) was exhonerated briefly (assassinated) as a king, and I feel is the inspiration for Seigfreid, the heroic dragonslayer from 1st century Gothic epics. Through this character and oral history, Hermann lived on to inspire German nationalism and defiance to the Roman Empire. The success of this battle led to a resurgence of German resistance to Roman occupation, and the eventual abandonment by Rome of conquering Germany. This is the infamous battle, was said to have driven Augustus Caesar partially mad, and he could be heard striking his head against doors and walls while mumbling or screaming "Varus! Give me back my legions!" Pretty heavy stuff if you consider just how powerful, sophisticated, and intelligent Augustus Caesar was.
In the short term, this had minor effects. The Roman Empire, under Augustus' heirs, tried vainly to fulfill the dream of conquering and civilizing Germany. Long story short- armies were amassed under various "Caesars", battles were fought, but mutinies and low morale led to an eventual stalemate on the issue, drawing the border between The Roman Empire and Germany at the Rhine. The Roman Empire set its sights on other areas, as it still had problems with Parthians, Illyrians, Judaea, and (very important to note) an upstart, problemic Jewish carpenter name Jesus who was starting quite the uproar in Israel.
it is important to note that the aforementioned German pride and nationalism had long term affects. This pride has been attributed to the ideals of Aryanism and its brain-child, Nazism. Richard Wagner used the stories of Seigfried for his operas "Der Ring des Nibelungen". These operas utilized the heroic character of Seigfried (remember, based on Hermann) and are peppered with symbols which would be embraced by the Nazis as Aryan, as well as his discrete anti-semitical words and very vocal distaste of Jews. Adolf Hitler would use these same operas and idealologies in his propogandas against the Jews, and he himself used Siegfried and Aryan ideals (not to mention, its Ariosophatic focusses, which are based in Teutonic superiority, which is another fancy term for Aryan Pride) as the foundation for German Nationalism and Jewish inferiority during the Holocaust.
From a military standpoint, it is important to note that had the Romans conquered Germany, it would have been occupied and "civilized", meaning it would have paid taxes to Rome and built up its own military for the defense of Germany and its Overlord, Rome. This did not happen. Instead, most of Germany lived autonomously, fighting amongst itself and the Romans and whomever happened to be around. The invasion of the Huns in the 4th Century AD sent the Goths fleeing to Rome for refuge, which, as we all know, led to a Gothic revolt in Rome and the fall of Rome to Gothic pillage.
So, why does this matter? How does this affect the world? Who cares?
Imagine if Varus hadn't been fooled, and his forces hadn't been massacred. Imagine if the defeated German tribes had broken their loose alliance and fled to their hovels, as the Romans came marching in with aqueducts, roads, government, and protection. There would not have been roots lain for the Aryan Race beliefs in regards to Germany. There would not have been a Siegfried, or Wagner's operas, or a foundation for Nazism and its occult. Perhaps there would not have been yet another supporter of anti-semitism in regards to Aryan Superiority? I know that is stretching it a bit, but hey, i'm reaching for straws at this point, as my attention span is short. There would have been a fortified and prepared Germany, a richer Rome, and no Hun invasion, in my opinion. There would not have come a Holy Roman Empire run by Germans and Franks, as divided a Europe, an independent Britainnia, and possibly we'd all be speaking a Romance language right now. It's a stretch, but I don't think one can rule it out.
Feel free to critique. This is my first post, so keep that in mind.