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.

But the men who led Europe into the First World War found it more comforting to look elsewhere — at battles where victory was swift and the enemy had little firepower. In 1914 Europe had not had a major war in more than 40 years and, except for the Russians, almost all officers who had actually seen combat had done so in lopsided colonial wars in Africa and Asia.