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The heir to the Austro-Hungarian throne, Archduke Franz Ferdinand and his wife Sophie were killed in Sarajevo by a Serbian nationalist named Gavrilo Princip. Princip was a member of a secret society called the Black Hand that was bent on freeing the Balkan states from Austro-Hungarian rule.
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Unsatisfied with Serbia's response to Austria's punishing ultimatum, Austria-Hungary declared war on Serbia.
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Russia, as Serbia's protector and ally, began mobilizing its troops in preparation for war.
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After a furious exchange of telegrams between Germany's kaiser Wilhelm and Russia's czar Nicholas (both cousins), Russia refused to stop mobilizing their troops and Germany declared war on Russia.
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The US President Woodrow Wilson officially proclaimed the US a neutral country to protect US overseas investments.
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The German embassy issued a warning urging overseas passengers traveling on British ships that there was a war taking place and British ships risked attack by the German navy.
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A German U-boat sank a British passenger liner off the coast of Ireland. 128 American citizens died. Germany argued there were munitions aboard and President Wilson called for an immediate end to unrestricted submarine warfare.
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The Germans launched a major offensive against the Allied forces in France. The battle, which ended in December 1916, resulted in nearly 1 million casualties. The French were able to hold their position, but at a massive loss in manpower.
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A German U-boat torpedoed a French passenger ship, the Sussex. Two Americans were aboard (not killed) and the US threatened to cut diplomatic ties with Germany.
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After protests by the American government, Germany promised to end the tactic of unrestricted submarine warfare
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The British military supported by other Allies launched an offensive near the Somme River in France to draw Central Power forces away from Verdun. Brutal fighting led to an estimated 1 million casualties on both sides. The British gained a mere 6 miles of territory and failed to reach their objective.
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German spies were accused of setting fires and detonating large explosions to sabotage the Black Tom depot and prevent the US from delivering the arms to the Allied Powers.
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Germany, in an effort to further inflict damage on the Allied Powers resumed their naval operations of unrestricted submarine warfare.
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The United States in response to Germany's resumption of unrestricted submarine warfare breaks off diplomatic relations with Germany and draws closer to World War I.
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The British intercepted a coded telegram urging Mexico to wage war against the United States in exchange for a return of the American Southwest the US had previously taken. The American and Mexican governments do not take the telegram very seriously, but it is a major propaganda victory for the Allies.
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Between March 16 and 18, Germany sank the City of Memphis, the Illinois, and the Vigilancia. By March 20, President Wilson's Cabinet had voted unanimously for war with Germany.
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Following Germany's resumption of unrestricted submarine warfare, Wilson gained approval for a declaration of war by Congress and signed the document, officially entering the US into World War I.
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Following the Bolshevik revolution, Russia led by Vladimir Lenin signed the Treaty of Brest-Litovsk allowing for Russia to withdraw from the war. Russia's withdrawal put additional pressure on the French and British and drove the US closer to the Allies.
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An armistice, or cease-fire is signed bringing an end to hostilities on the 11th hour of the 11th day of the 11th month. The two sides will now have to negotiate a peace arrangement.
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The European powers signed the Treaty of Versailles, officially ending the war between them. The US never signed the treaty and arranged separate peace agreements with the warring nations.
