The eleventh hour of the eleventh day of the eleventh month
Almost all of modern history has transpired as the result of a single gunshot fired in Sarajevo in 1914.
When the Serb nationalist Gavrilo Princip assassinated the Hapsburg Archduke Ferdinand, Bismarck's famous prediction came true: that a catastrophic war would engulf all of Europe, and that it would start "over some damn foolish thing in the Balkans."
Neither Bismarck, Princip, nor any of the decision-makers of that day could have realized how far-reaching the assassination's consequences would be. The War To End All Wars did nothing of the sort; although the guns did finally fall silent in the eleventh hour of the eleventh day of the eleventh month in 1918, the fallout from that war would ultimately lead to a full century of dealing with its consequences.
Most of 1919 was occupied by the world's leaders congregating in Paris to hammer out a peace agreement; the conference ended with a major redrawing of the borders of Europe, Africa, and the Middle East, often in an arbitrary and random way. The new nations created out of that conference bear names which have resonated in the conflicted history of the ensuing generations: Yugoslavia, Iraq, and Saudi Arabia are only three of the nations which didn't exist prior to the Paris conference and which have played significant roles in the instability of the modern age.
Further, the punitive and draconian measures taken against Germany in 1919 directly caused the rise of Nazism and indirectly led to the Great Depression. Those two events, in turn, led to World War II (which was really nothing more than a continuation of World War I), the Cold War, and the post-Cold-War era in which we currently find ourselves.
Although Iraq is currently on the center stage of global conflict, the broader problems caused by the 1919 Paris conference remain: nations were created out of thin air, based on little more than scribblings on a map drawn to satisfy the short-term urges of power held by the decision-makers of that time. Modern peacemakers seem no more forward-thinking than their counterparts from 88 years ago. Decisions made today regarding the Middle East (and other parts of the world) are likely to resonate for generations to come, and with outcomes just as unpredictable.
That's the real lesson of Armistice Day (as it was once called): long-term consequences of political decisions are unpredictable and can easily lead to a destabilized world.