A few days back was the Century anniversary of a big event.
That event was the most world-changing instance of use of a firearm since the dawn of the age of mass-produced firearms.
Gavril Princip fired two shots at Archduke Ferdinand of Austria on the 28th day of June, 1914. Princip took advantage of a chance opportunity, after playing the role of observer in a failed assassination attempt (by bomb) earlier that day.
Ferdinand died shortly afterwards. Princip's cause (more independence for Serbia) was not helped by this assassination. A declaration of war ensued. A chain of allied nations mobilized their armies, rattles sabers, and discovered that they couldn't honorably back down from warfare. Especially if their ally had already declared war on a mutual opponent. This chain of mobilizations led to warfare among the richest, most powerful nations of the world.
The powers of Europe that fought each other had colonies and navies around the world. The Great War was also dubbed The World War. (One possible theater of war did not develop. Mexico was encouraged by Germany to open war against the U.S., in a manner that would have made the world-wide nature of the war more obvious. Mexico declined, but the message was decoded and seen by the American public. This message, plus the unrestricted submarine warfare of the German navy, led to U.S. entry into the War.)
Most of the international politics of the 20th Century (and early 21st Century) can be traced to the reshaping of maps, empires, and governments that occurred during and after the Great War.
The dissolution of the former Ottoman Turkish Empire, the genocide of Armenians in Turkey, the rise of Communism in Eastern Europe, the troubles in Germany that gave rise to the Third Reich, the replay of the European War, the Imperial ambitions of Japan, the de-colonization of Africa and the Middle East, the re-alignment of world power to include the United States as a major player, the rise of militant Islam in Middle East...most of these trace back to something that was done during the Great War or in the treaties that followed it.
It is likely that the chain of alliances and military mobilization might have been triggered by another event that decade, had Princip not taken the shots he did.
But the details of the how and the why are lost in the dreams of alternate history.
And I wonder how long the current world order will last, and when (or whether) a single event will trigger a similar cascade.
UPDATE: found a series of maps and diagrams that help explain the entire chain of events.