World War I

Encouraged by German Emperor Kaiser Wilhelm II, Austria-Hungary declares war on Serbia. The declaration triggers a rapid chain reaction of military mobilizations across Europe.

A German submarine sinks the British ocean liner RMS Lusitania, killing nearly 1,200 people. US President Woodrow Wilson issues severe diplomatic warnings to Germany regarding unrestricted submarine warfare.

British Field Marshal Douglas Haig launches a joint offensive with French forces along the Somme River. The first day results in 57,000 British casualties, marking one of the deadliest days in military history.

Spurred by unrestricted submarine attacks and the Zimmermann Telegram, President Woodrow Wilson leads the United States into World War I on the Allied side.

The Russian Bolshevik government under Lenin signs a peace treaty with the Central Powers, formally exiting the war and surrendering large territorial concessions to Germany.

Allied leaders Woodrow Wilson, David Lloyd George, and Georges Clemenceau finalize the peace treaty imposing heavy reparations and territorial losses on Germany.