On 11/11/1918 World War I’s armistice went into effect.
A good argument is that the European Civil War that was a part of actually ended on 1/1/1999, when Germany and France were united by a common currency.
The only leader who appeared to believe in democracy in World War I was the Austro-Hungarian Emperor Karl the Blessed. World War I helped drive him off his throne.
War is an important tool of policy in a system of international anarchy. But in the 20th century, which saw at its height Hitler, Stalin and Mao, the War that today commerates led to a man who might have been a Saint losing an Empire in the middle of Europe.
Catholicgauze notes that the sacrifices of the soldiers in the Great War were depressingly wonderful. With the exception of Americans, who ended the fighting, the Europeans fought to save their empires and they ended by destroying their continent.
While by many measures the twentieth century was the most peaceful and progressive in all of human history, it nonetheless was one of great lost opportunities. The political-economic-financial-military union of the EU and NATO has brought peace to Europe, and the rise of China and India — combined with the continued strength of the United States — may bring peace to the world as well.