In 1914, European powers were divided into rival alliances: the Triple Entente (France, UK, Russia) and the Triple Alliance (Germany, Austria-Hungary, Italy). The assassination of Archduke Franz Ferdinand in Sarajevo on June 28 by a Serbian nationalist triggered a series of ultimatums and declarations of war. Germany's Schlieffen Plan aimed for a swift victory over France before turning to Russia. Italy and the United States initially remained neutral, but British entry was solidified when Germany invaded neutral Belgium.

The German advance into Belgium faced strong resistance, and atrocities against Belgian civilians spurred Allied propaganda. French offenses and British forces at Mons were pushed back, leading to a desperate stand at the Marne that saved Paris. The 'Race to the Sea' and the First Battle of Ypres resulted in entrenched positions along a 500 km front, marking the beginning of trench warfare. Naval battles saw British dominance, imposing a blockade, while German U-boats emerged as a new threat. On the Eastern Front, Russia suffered heavy losses at Tannenberg and Masurian Lakes against Germany, while Austria-Hungary struggled against Serbia and Russia.

The war extended beyond Europe, with fighting in European colonies in Africa and Asia. Japan declared war on Germany and occupied German naval bases. German forces under von Lettow-Vorbeck in East Africa conducted a successful guerrilla campaign. Naval combat in the Pacific saw both British and German victories and losses. In the Middle East, British forces secured vital Persian oil access. The end of 1914 saw a brief Christmas truce. By January 1915, over a million soldiers had fallen. Germany initiated strategic bombing with Zeppelins and declared naval war zones around Britain, aiming to starve the country. Allied offensives like Neuve Chapelle incurred heavy losses for minimal gains. The Germans introduced poison gas at the Second Battle of Ypres. The Gallipoli campaign, aimed at taking Constantinople, resulted in a costly stalemate for the Allies, including ANZAC forces. The Ottoman Empire began the systematic deportation and murder of Armenians. Italy joined the Allies in May 1915, opening a new front against Austria-Hungary along the Isonzo River. The great Russian retreat saw the fall of Poland to Germany. The sinking of the Lusitania by a German U-boat outraged the US. The Fokker Eindecker brought German air superiority. By year-end, the Allies abandoned the costly Gallipoli campaign. 1915 was a difficult year for the Allies with no tangible gains, but preparations for larger offensives in 1916 began.

By 1916, the war had become an industrial conflict with millions of casualties. Naval blockades led to food and fuel shortages. Austria-Hungary attacked Montenegro, forcing its surrender. On the Caucasus front, Russia launched a successful winter offensive against the Ottomans. Germany's siege on Verdun aimed to 'bleed France white,' becoming one of the most brutal battles. Britain introduced conscription and steel helmets. Portugal joined the Allies. Russia's offensive at Lake Naroch to relieve pressure on Verdun was a disaster. The British surrender at Kut in Mesopotamia was a major defeat. Britain and France secretly signed the Sykes-Picot Agreement to divide the Middle East, while Hussein bin Ali led an Arab revolt against Ottoman rule. The Battle of Jutland was the only major naval battle, where the British suffered more losses but the German fleet retreated. The summer of 1916 saw major Allied offensives: Russia's Brusilov Offensive on the Eastern Front achieved significant gains but at high cost. The Battle of the Somme, initiated by Britain and France, was a disaster on its first day for the British, with 57,000 casualties. Tanks were introduced but had little impact initially. Romania joined the Allies but was swiftly invaded. Verdun finally ended, with enormous casualties on both sides. The Brusilov Offensive concluded with catastrophic losses for both Russia and Austria-Hungary. The Somme concluded with huge casualties for the Allies and Germans. Romania was overrun. Political leadership changes occurred in the UK and France, and US President Wilson's peace efforts failed.

Germany, facing long-term resource disadvantages, resumed unrestricted submarine warfare to starve Britain, risking US entry. The sinking of the SS Housatonic and the Zimmermann Telegram (exposing German plans to encourage Mexico to attack the US) pushed the US closer to war. In Russia, the February Revolution led to the Tsar's abdication and the continuation of the war by a provisional government, but Russian troops began deserting. The US declared war on Germany in April, bringing vast resources, but mobilization would take time. German U-boats had their most successful month in April. On the Western Front, the Battle of Arras, including the Canadian capture of Vimy Ridge, was a costly diversion. Air warfare intensified, with German air superiority leading to 'Bloody April.' French General Nivelle's main offensive was a costly failure, leading to mutinies in the French army and Nivelle's replacement by Pétain. The British achieved local success with mining operations at Messines Ridge. Greece joined the Allies. The Russian July Offensive was a disastrous failure, leading to widespread desertion. The Allies adopted convoy systems to counter U-boats, turning the tide in the submarine war. Germany's Reichstag passed a 'Peace Resolution,' ignored by the military high command. The Third Battle of Ypres (Passchendaele) became a muddy, costly stalemate for the British. On the Italian Front, exhaustion set in after the 11th Battle of Isonzo. More nations like Brazil and China joined the Allies. In the Middle East, British forces captured Baghdad and advanced towards Jerusalem, which fell in December. On the Italian front, German reinforcements from the East helped break the Italian army at Caporetto. In Russia, the October Revolution brought Lenin's Bolsheviks to power, determined to end the war. Georges Clemenceau became French Prime Minister, advocating total war. However, the Allies were in crisis, with Russia out of the war and French and Italian armies weakened. The British launched the Battle of Cambrai, the first major tank assault, with initial success but ultimately limited gains. Finland declared independence, and Romania and Russia signed armistices with the Central Powers, effectively ending the Eastern Front.

With Russia out, Germany could transfer troops to the Western Front for a final offensive. The US entry, however, brought new manpower and resources for the Allies. President Wilson announced his 'Fourteen Points' for post-war peace. The Treaty of Brest-Litovsk solidified Russia's exit, freeing up half a million German soldiers for the Western Front. Ludendorff's Spring Offensive aimed for a decisive victory, initially surprising the Allies and pushing back the British Fifth Army. Ferdinand Foch was appointed Supreme Commander of Allied forces. The German advance was eventually halted near Amiens by British and Australian troops. Air superiority shifted to the Allies due to greater resources. Manfred von Richthofen, the 'Red Baron,' was shot down. The British began strategic bombing campaigns against German cities. Ludendorff's offensives continued towards Paris, but new American divisions, including the 1st and 2nd Divisions, played a crucial role in stopping them at Cantigny and Belleau Wood. Austro-Hungarian attacks on the Italian Front failed. Allied intervention in the Russian Civil War began. The Second Battle of the Marne saw the final German offensive defeated. Ludendorff's offensives, while inflicting heavy casualties, failed to achieve a decisive breakthrough. The Allies then launched the 'Hundred Days Offensive,' starting with the Battle of Amiens, which saw significant gains supported by tanks and aircraft. The German army was exhausted and began surrendering en masse. Trench warfare ended as the Germans retreated. In the Balkans, an Allied offensive broke Bulgarian positions, leading to their armistice. In the Middle East, British-led forces defeated the Ottomans at Megiddo and occupied Damascus and Aleppo. On the Western Front, the Allied general attack led to the breach of the Hindenburg Line. Ludendorff informed the Kaiser that the military situation was hopeless, and Germany sought an armistice based on Wilson's demands, including the Kaiser's abdication. On the Italian Front, the Battle of Vittorio Veneto led to the collapse of the Austro-Hungarian army. The Ottoman Empire and Austria-Hungary signed armistices. Naval mutinies spread revolutionary sentiment in Germany, leading to the Kaiser's abdication and the proclamation of a German Republic. On November 11, 1918, the armistice was signed in Compiègne, taking effect at 11:00 AM. Henry Gunther, an American soldier, was the last to die just before the armistice. In East Africa, General von Lettow-Vorbeck, undefeated, surrendered his forces three days later.

The Paris Peace Conference at Versailles led to the creation of the League of Nations. The Treaty of Versailles imposed harsh terms on Germany: military restrictions, war reparations, territorial losses, colonial confiscation, and the 'war guilt clause.' Europe's borders were redrawn, with Poland re-emerging and new nations like Czechoslovakia and Yugoslavia forming from the Austro-Hungarian Empire. The Ottoman Empire was dismantled, and new states were created in the Middle East under European control, sowing seeds for future conflicts. German possessions in China were given to Japan, angering China. The First World War resulted in 9.5 million military deaths, 21 million wounded, and 7 million civilian deaths. Vast areas of Europe were devastated. Ancient empires fell, new states emerged, and the world was irrevocably changed.