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Importance: The murder of Archduke Franz Ferdinand by Gavrilo Princip set off a chain reaction of ultimatums and alliances that quickly escalated into World War I. It provided Austria-Hungary the pretext to confront Serbia, which triggered mutual-defense commitments across Europe and led to full-scale war. -
Importance: At the war’s outbreak the United States declared neutrality, hoping to avoid European entanglement and to continue trade with all sides; that stance defined U.S. policy until events later forced intervention. -
Importance: The Allied victory at the Marne stopped the German advance on Paris and ended hopes of a quick German victory in the west, leading instead to the long trench warfare stalemate that defined most of WWI. -
Importance: The German submarine attack that sank the British passenger liner Lusitania killed many civilians (including Americans) and enraged public opinion in the U.S., increasing anti-German sentiment and helping push the U.S. closer to war. -
Importance: Verdun became a symbol of brutal, grinding attrition; the long German attack and French defense caused enormous casualties and exemplified the destructive human cost of trench warfare. -
Importance: German attacks on passenger and neutral ships like the Sussex led to diplomatic protests from the U.S. and temporary German promises to restrict submarine warfare—promises that were later broken and helped move the U.S. toward war. -
Importance: The Somme was one of the bloodiest Allied offensives, producing huge casualties for small territorial gains and demonstrating the limits of mass infantry attacks against entrenched defenses. -
Importance: Germany’s secret proposal to Mexico offering alliance and U.S. territory if Mexico attacked the United States was intercepted and published, shocking Americans and helping build public support for U.S. entry into the war. -
Importance: Germany’s resumption of unrestricted U-boat attacks on all ships—neutral and belligerent—made transatlantic shipping dangerous, sunk U.S.-bound vessels, and was a principal reason the U.S. decided war was necessary. -
Importance: When Congress declared war on Germany in April 1917, American manpower, industrial production, and financial resources began to flow to the Allied cause—this U.S. involvement helped tip the balance and make an Allied victory possible. -
Importance: The Selective Service Act created a national draft that quickly expanded the U.S. Army, supplying the troops needed to form the American Expeditionary Force and sustain overseas operations. -
Importance: The Espionage Act criminalized many forms of dissent or interference with the draft and war effort, giving the government broad power to suppress criticism and control wartime information. -
Importance: The arrival of U.S. troops and later large-scale American offensives supplied fresh manpower and morale to exhausted Allied forces and played a decisive role in the final 1918 offensives. -
Importance: Wilson’s Fourteen Points outlined a vision for a just peace and self-determination after the war (including a proposed League of Nations), shaping diplomacy at the Paris Peace Conference even as many Allies disagreed with his ideals. -
Importance: The influenza pandemic infected millions worldwide (including troops), killed far more people than many battlefield campaigns, and disrupted military operations and civilian life during and after the war. -
Importance: Following the Russian Revolution, the Bolshevik government negotiated the Treaty of Brest-Litovsk and exited the war, freeing German forces on the Eastern Front to transfer troops westward (briefly strengthening Germany’s position). -
Importance: The Sedition Act expanded restrictions on speech and dissent beyond the Espionage Act, further curbing criticism of the government and wartime policies and affecting civil liberties at home. -
Importance: The Meuse–Argonne offensive was one of the largest American operations of WWI and a major component of the final Allied push that broke German lines and hastened the end of the war. -
Importance: The armistice signed in the Compiègne area took effect at 11:00 a.m. on November 11, 1918, ending fighting on the Western Front and marking the close of hostilities in World War I. -
Importance: The Treaty of Versailles formally ended WWI but imposed heavy reparations, territorial losses, and military limits on Germany; the harsh terms and unsettled political outcomes contributed to political instability in Europe in the decades that followed.