The world war that ended in 1945 was one of the biggest ever. It was fought by nations on all six continents, and involved nearly half of the planet’s population. But why did it come about? Many historians blame a combination of factors, including the assassination of Archduke Franz Ferdinand, which triggered a series of political, diplomatic and military events that turned a local conflict in South-East Europe into a global war.

The assassination of the heir to the Austro-Hungarian throne in Sarajevo set off a chain reaction. Within weeks, the continent’s largest armies were mobilising, and new countries joined them seemingly every week. The speed with which this happened was extraordinary. Diplomats and governments had missed all kinds of opportunities to slow down the mobilisation. They couldn’t keep up with a trend that was accelerating all the time.

Adding to the escalation was the way the major powers were linked by an intricate web of alliances. The theory behind these alliances was that a stronger power would be less likely to attack a weaker one if it had the support of a powerful ally. In practice, the alliance networks dragged in allies of different types and nationalities, making the crisis much bigger than the two powers that started it.

At first, the United States tried to remain neutral. But public opinion began to shift after Germany’s sinking of the passenger ship Lusitania in May 1915, which killed almost 1,200 people, including hundreds of Americans. And in February 1917, Congress passed a bill that made the country ready to go to war against Germany.