TURNER, Me. — ONE hundred years ago today, Austro-Hungarian artillery and gunboats on the Danube began shelling Serbia — the first shots of the great cataclysm that over the next four and a half years would remake our world for the worse, in every conceivable way. We think of the First World War as having its causes in Europe, where the greatest bloodshed and destruction would take place. But several of the illusions that propelled the major powers so swiftly into war had their roots in far corners of the world.

The biggest illusion, of course, was that victory would be quick and easy. “You will be home,” Kaiser Wilhelm II of Germany told his troops, “before the leaves have fallen from the trees.” The German campaign plan called for knocking France out of the war in 42 days. The Allies were not quite so arrogant, but were confident of triumph in months, not years.

A second illusion of those who marched proudly into battle in 1914 was that they would be shooting at the enemy, but that he would not be shooting back, or at least not effectively. How else to explain that most soldiers on both sides had no metal helmets? And that millions of French infantrymen, as well as the Austro-Hungarian cavalry, wore combat uniforms of brilliant red and blue? As the war began, troops from both sides advanced over open ground en masse, as if they were not facing repeating rifles and machine guns: bayonet charges by the French, and ranks of young Germans walking, arms linked, toward astonished British soldiers. The British would make plenty of similar suicidal advances of their own in the years ahead.

Where were these illusions born? They came from the way generals cherry-picked previous wars to learn from. A close look at the siege of Petersburg, Va., in the American Civil War, for instance, would have provided a lesson in trench warfare — and a sense of what it meant to be under fire from an early ancestor of the machine gun, the Gatling gun. A similar foretaste of both trench warfare and the power of the machine gun could be had by studying the siege of Port Arthur (now Dalian, China) in the Russo-Japanese War of 1904-5.