Franz Ferdinand assassinated Austrian Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire, and his wife are assassinated in Sarajevo by Serbian nationalist Gavrilo Princip after Bosnia-Herzegovina is annexed into Austria.

Austria declares war on Serbia When the Austro-Hungarian Empire declares war on Serbia in response to the assassination of the archduke, a series of alliances quickly brings Germany, Russia and France into the conflict.

Germany declares war on France After declaring war on Russia on 1 August, Germany declares war on Russian ally France, then invades neutral Belgium on 4 August to attack France. The invasion brings the British Empire into the war.

Battle of Tannenberg German forces surround and effectively destroy the Russian Second Army as they march into Prussia. They are helped by intercepted unencrypted Russian messages and the animosity between the Russian generals leading the First and Second Armies.

Battle of the Dardanelles The British begin a series of naval attacks against Turkish forts in the Dardanelles. The ill-fated Gallipoli campaign will follow later in the spring.

Germans sink the Lusitania A German U-boat (submarine) sinks the British passenger liner the Lusitania with the loss of almost 1,200 lives, including 128 Americans. The event rouses anti-German sentiment and will help bring the United States into the war.

Italy joins the Allies Italy declares war on Austria-Hungary and enters the war on the side of the Allied Powers.

Battle of Verdun The Germans launch an attack against the French at Verdun. Verdun turns into the longest battle of the war: fighting will continue for nine months and result in approximately 1 million casualties.

Battle of Jutland In the largest naval battle of WWI, German battleships attempt to break the British blockade. The battle continues on 1 June, and while the British sustain heavier losses, the Germans High Seas Fleet withdraws and will mostly remain in harbor for the duration of the war.

Battle of the Somme The British prepare for this battle with a weeklong artillery barrage to weaken German defenses but still suffer almost 60,000 casualties the first day, with almost a third of these killed. The Battle of the Somme will continue into November and result in another million casualties.

First airplane raid on London The Germans launch the first airplane raid on London, hoping to busy the British air force defending the home front. The plane crashes on its return trip, and the crew is captured by the Allies.

German U-boat campaign Germany increases its U-boat activity, resuming unrestricted submarine warfare around Great Britain and sinking allied and neutral ships on sight. The sinking of American ships will help propel the United States into the war.

United States enters the war On 2 April President Woodrow Wilson asks Congress for a declaration of war against Germany, arguing that "the world must be made safe for democracy." On 6 April the United States declares war.

Treaty of Brest-Litovsk After the successful Bolshevik Revolution, Russia ends hostilities with Germany and withdraws from the war.

Wilson’s 'Fourteen Points' speech In a speech before the U.S. Congress, President Woodrow Wilson explains his 14-point plan for lasting world peace. These include no secret treaties, reduction in armaments, freedom of the seas and plans for the restoration of countries and territories affected by the war.

Red Baron shot down Manfred von Richthofen, the Red Baron, is shot down after shooting down 80 Allied planes himself. He is buried by his enemies with military honors.

Meuse-Argonne Offensive Also known as the Battle of the Argonne Forest, this is the largest operation undertaken by — and greatest victory for — the American Expeditionary Force in World War I. It is also the deadliest. The next day, combined Allied forces will break through German defensive fortifications along the Hindenburg Line in northeastern France.

Kaiser Wilhelm II abdicates With the collapse of Germany’s Imperial government, Kaiser Wilhelm II abdicates and goes into exile in Holland. In Germany, a republic is formed.