How World War I Started: The Chain of Events After Sarajevo

The World Before the Shots Were Fired

To understand why one assassination triggered a global war, you need to understand the volatile structure of European power in 1914. The continent was divided into two heavily armed alliance blocs: the Triple Entente, comprising France, Russia, and Britain, and the Triple Alliance (or Central Powers), comprising Germany, Austria-Hungary, and Italy. These alliances were designed to deter war through mutual deterrence, but they had a dangerous built-in feature: a conflict between any two members would automatically pull in the others.

Beneath the alliance structure lay a decade of escalating tensions. Germany had been rapidly industrializing and building a massive navy, alarming Britain. France nursed deep resentment over its defeat in the Franco-Prussian War of 1870-71 and the loss of Alsace-Lorraine. Russia was expanding its influence in the Balkans, which conflicted with Austro-Hungarian interests. The Ottoman Empire was collapsing, creating a power vacuum that every major power wanted to fill. Two Balkan Wars in 1912 and 1913 had already raised the temperature. Europe was a room full of gunpowder. Sarajevo provided the spark.

June 28, 1914: The Assassination

Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, arrived in Sarajevo, the capital of Bosnia, on June 28, 1914. Bosnia had been annexed by Austria-Hungary in 1908, and this annexation was deeply resented by Serbian nationalists who saw Bosnia as territory that should belong to a greater South Slavic state. A group called the Black Hand, connected to Serbian military intelligence, had planned an assassination attempt.

The first attempt that morning failed: a grenade thrown at the archduke's car bounced off and exploded under a following vehicle. Franz Ferdinand continued to his scheduled appointment. Later, while traveling to visit those injured in the earlier attack, his driver took a wrong turn. Correcting the mistake, he stopped the car almost directly in front of 19-year-old Gavrilo Princip, one of the conspirators who had given up hope for the day and was standing at a deli. Princip fired twice, killing Franz Ferdinand and his wife Sophie. The immediate cause of the first world war was, in part, a wrong turn.

Austria-Hungary's Ultimatum to Serbia

Vienna blamed Serbia for the assassination, regardless of the extent of official Serbian government involvement. Austria-Hungary's leaders saw this as an opportunity to crush Serbian nationalism once and for all. Before acting, they sought assurance from Germany that Berlin would back them. Kaiser Wilhelm II and German Chancellor Bethmann Hollweg gave what became known as the blank check, unconditional support for whatever Austria-Hungary decided to do.

On July 23, Austria-Hungary delivered an ultimatum to Serbia with deliberately humiliating terms designed to be unacceptable. Among its demands were that Austro-Hungarian officials participate directly in Serbia's investigation and that Austria-Hungary approve the outcome. Serbia's reply, sent on July 25, accepted nearly all terms but objected to direct Austro-Hungarian participation in the investigation as a violation of sovereignty. Vienna declared this insufficient and broke off diplomatic relations. The diplomatic window was closing fast.

The July Crisis: Five Weeks That Changed the World

The period from the assassination to the outbreak of general war is known as the July Crisis. What followed was a catastrophic failure of diplomacy in which each actor's decisions, constrained by alliance obligations, military timetables, and domestic political pressures, narrowed the space for peace until it disappeared entirely.

• July 28: Austria-Hungary declares war on Serbia. Russia begins mobilizing its vast army in support of Serbia.
• July 31: Germany demands Russia stop mobilizing within 12 hours and demands that France declare neutrality. Both demands are impossible to meet.
• August 1: Germany declares war on Russia. France begins mobilization.
• August 3: Germany declares war on France and invades neutral Belgium, following the Schlieffen Plan, a pre-existing strategy to knock out France quickly before turning to face Russia.
• August 4: Britain declares war on Germany, citing the violation of Belgian neutrality guaranteed by a treaty from 1839. The war is now general across the major powers.

The Schlieffen Plan and Military Logic

One of the least appreciated drivers of the war's outbreak was the inflexibility of military mobilization plans. Germany's war strategy, the Schlieffen Plan, assumed a two-front war against France and Russia and required Germany to knock France out within six weeks before the slower Russian army was fully deployed. This plan demanded invading Belgium to outflank French defenses, and it required mobilization to begin immediately at the first sign of war with Russia.

This military logic dramatically reduced political flexibility. When the Kaiser briefly considered a last-minute alternative of fighting only Russia in the east, his military chief of staff Helmuth von Moltke told him it was impossible: trains had already been rerouted, supplies had been positioned, and reversing the plan mid-execution would produce chaos. Military timetables had effectively overridden diplomatic options. Similar rigidities applied to Russia, whose mobilization plan did not distinguish between partial and full mobilization.

Who Was Responsible?

Historians have debated responsibility for World War I for over a century, and no fully settled consensus exists. The Treaty of Versailles in 1919 placed sole blame on Germany and its allies in the infamous war guilt clause, Article 231, which provided the legal basis for reparations. Most contemporary historians reject this view as too simple.

Germany bears significant responsibility for the blank check that encouraged Austria-Hungary's aggression and for the Schlieffen Plan's invasion of Belgium. Austria-Hungary bears responsibility for issuing a deliberately unacceptable ultimatum to Serbia. Russia bears responsibility for premature full mobilization, which accelerated German military responses. Britain and France share responsibility for insufficiently signaling to Germany that they would fight, potentially deterring German aggression, while also failing to rein in Russia's mobilization.

Why It Became a World War

The conflict might have remained a regional Balkan war had the alliance system not turned bilateral conflicts into multilateral ones automatically. Once Austria-Hungary and Russia were at war, Germany was obligated to join. Germany's invasion of Belgium brought Britain in. French treaty obligations with Russia and Britain brought France in. Ottoman entry in November 1914 extended the war to the Middle East. Japan, bound by a separate treaty with Britain, seized German colonies in Asia and the Pacific. Italy, despite being in the Triple Alliance, initially stayed out and then joined the Entente in 1915 when offered territorial concessions.

The war that was expected to be over by Christmas 1914 instead ground on for four years, killing approximately 20 million people. The lesson that statesmen and historians drew from its outbreak shaped international relations for the rest of the century: that alliance systems with automatic triggers are dangerous, that military planning can constrain political decisions fatally, and that crises require communication channels strong enough to slow the momentum toward catastrophe. Whether those lessons were truly learned is a question that each subsequent generation has had to answer anew.