When historians look back on the world war that killed 9 million people, they tend to focus on the summer of 1914 and ask whether it could have been stopped. But the deeper causes of the war are just as important to its long-term legacy.

At the time, nations had few tools for stopping a conflict once it started; diplomatic systems were outdated and the pace of military mobilization was accelerating. The combination of these factors pushed states to the brink, allowing events to spiral out of control and lead to the outbreak of a global war with terrible consequences for millions of men, women, and children.

The assassination of Archduke Franz Ferdinand of Austria on 28 June 1914 set off a chain reaction that led to war between Austria-Hungary and Serbia, joined by Germany, Russia, and France. The French government, worried about the potential for a two-front war, agreed to the Sykes–Picot Agreement with Britain and Russia in July, dividing the Ottoman Empire into French, British, and Russian spheres of influence, which would have long-term ramifications.

On the western front, the stalemate created by bloody attrition turned the landscape of northern Europe into a lunar mud-scape of shell craters, corpses, abandoned equipment, and wire entanglements, with commanders struggling to break the deadlock with costly offensives. The stalemate lasted until an Allied breakthrough finally ended the war in 1918.

The major powers that started the war lost their empires, and many of them would be destabilized for years afterward. Yet they continued to fight, even after it was clear that the conflict was not in their own self-interests.