A century ago, millions of young men left the security of their homes and went off to war. They went eagerly, swept along by a wave of patriotism. “I am happy and full of excitement over the wonderful days ahead,” wrote an American volunteer in 1914.

Soon, though, their enthusiasm turned to bitterness. No one foresaw the way those huge armies would get bogged down for years in the mud of Belgium and France. At the time, people termed it the “Great War.” Today, we know it as the first world war.

The first world war was decidedly great in terms of casualties. By some estimates, it left about 10 million dead and 20 million mutilated. It was also the result of great blunders. European statesmen were unable to stop international tensions from escalating into a global conflict. More important, perhaps, is the fact that the “Great War” left great scars. It changed the world in ways that still affect us today.

The first world war broke out because of miscalculations. European leaders acted like a “generation of sleepwalkers that stumbled unawares over the ledge of doom during that halcyon summer of 1914,” explains the work The Fall of the Dynasties​—The Collapse of the Old Order 1905-1922.

Within weeks, the assassination of an Austrian archduke plunged all the major European powers into a war that they did not want. “How did it all happen?” the German chancellor was asked a few days after hostilities began. “Ah, if only one knew,” he sadly replied.

The leaders who made the fateful decisions that led up to the war had no inkling of the consequences. But reality soon dawned on the soldiers in the trenches. They discovered that their statesmen had failed them, their clergy had deceived them, and their generals had betrayed them. How so?

The statesmen promised that the war would open the way to a new and better world. The German chancellor proclaimed: “We are fighting for the fruits of our peaceful industry, for the inheritance of a great past, and for our future.” American President Woodrow Wilson helped to coin a reassuring popular slogan that the war would “make the world safe for democracy.” And in Britain, people thought it would be “a war to end war.” They were all mistaken.

The clergy supported the war enthusiastically. “The guardians of God’s word led the martial chorus. Total war came to mean total hatred,” states The Columbia History of the World. And clerics fanned rather than quenched the flames of hatred. “Clergymen were unable, and for the most part unwilling, to place Christian faith before nationality,” observes A History of Christianity. “Most took the easy way out and equated Christianity with patriotism. Christian soldiers of all denominations were exhorted to kill each other in the name of their Saviour.”

The generals promised a quick and easy victory, but it was not to be. Before long, the opposing armies came to a grueling stalemate. Thereafter, millions of soldiers faced what one historian described as “perhaps the cruelest large-scale ordeal that the flesh and spirit of man have endured.” Despite appalling losses, generals kept throwing their men against barricades of barbed wire and barrages of machine-gun fire. Not surprisingly, widespread mutinies broke out.

How did the first world war affect society? One historical work quotes a veteran as saying: “The war . . . scorched the minds and character of a generation.” Indeed, in the wake of that war, entire empires disappeared. That tragic conflict proved to be the prelude to the bloodiest century mankind has ever known. Revolutions and strikes came to seem almost commonplace.

Why did the war turn the world upside down? Was it really just a colossal accident? Do the answers reveal anything about our future?