The paradigmatic war of the last century began in popular enthusiasm for guns and glory, slogged without unequivocal resolution through trenches of mud and mass slaughter, and ended (perhaps paused) on at 11:00am/11/11 with European civilization in shreds and the seeds of further violence deeply sown among peoples bloodied and deranged.
The first World War produced some of history's first combat footage. Here's one of the better samples I was able to turn up.
The Great War all seems so long ago -- but both my parents were alive and easily old enough to remember that war. It gave shape to what historian Eric Hobsbawm called "short 20th century" from 1914-1989. Current upheavals in the Arab world take place within boundaries set without consent of the governed in a settlement imposed by its victors. The echoes are still with us.
In the United States, thousands of miles away and with millions less killed in that conflict, it is easy to forget. We call November 11 "Veterans Day" (not "Armistice Day" as in Britain or "Remembrance Day" as in Canada) and cheer for soldiers in fatigues at football games.
I still believe understanding World War I matters for building peace in this time and have written extensively about it: here, here, here, here, here, and here.