World War I (28 July 1914–11 November 1918) was a global conflict between the Allied Powers and the Central Powers that mobilized more than 60 million soldiers, introduced large‑scale industrialized warfare, and resulted in an estimated 15–22 million deaths and roughly 40 million total military and civilian casualties. Encyclopaedia Britannica; Encyclopaedia Britannica, Key Facts.

Origins and outbreak

The immediate catalyst was the assassination of Archduke Franz Ferdinand in Sarajevo on 28 June 1914 by Gavrilo Princip of Young Bosnia, which precipitated the July Crisis among the European great powers. Encyclopaedia Britannica. Austria‑Hungary delivered a stringent ultimatum to Serbia on 23 July; Serbia’s partial acceptance was deemed insufficient, and Austria‑Hungary declared war on 28 July 1914. HISTORY; Encyclopaedia Britannica.

Underlying structural causes included militarism, rival alliance blocs, imperial rivalries, and nationalist tensions, with German war planning influenced by the Schlieffen Plan for a rapid offensive through Belgium to defeat France before Russia could fully mobilize. Encyclopaedia Britannica; Encyclopaedia Britannica; [Hew Strachan, The First World War](book://Hew Strachan|The First World War|Penguin|2004); [Christopher Clark, The Sleepwalkers](book://Christopher Clark|The Sleepwalkers: How Europe Went to War in 1914|Harper|2013).

Belligerents and strategy

The Central Powers—Germany, Austria‑Hungary, the Ottoman Empire, and Bulgaria—fought the Allies, which initially included France, the United Kingdom, and Russia, and later Italy (from 1915), the United States (from 1917), Japan, and many dominions and colonies. Encyclopaedia Britannica. German operations executed a modified Schlieffen Plan that violated Belgian neutrality and brought the United Kingdom decisively into the war, while on the Eastern Front Russia’s early offensives were checked by German counterstrokes. Encyclopaedia Britannica; Encyclopaedia Britannica.

Major theaters and campaigns

On the Western Front, entrenched positional warfare dominated after the First Battle of the Marne (1914), culminating in attritional battles such as Verdun (February–December 1916) and the Battle of the Somme (July–November 1916). Verdun cost about 400,000 French and 350,000 German casualties, with some 300,000 killed; the Somme saw around 60,000 British casualties on 1 July alone and more than a million total casualties by November. Encyclopaedia Britannica; Encyclopaedia Britannica; Encyclopaedia Britannica.

In the Middle East, the Allies attempted to force the Dardanelles in the Gallipoli Campaign (1915–1916), an amphibious operation that failed with over 200,000 Allied casualties and comparable Ottoman losses; the campaign reshaped Allied politics and strategy. Encyclopaedia Britannica. Secret wartime diplomacy, including the Sykes–Picot Agreement (1916), outlined postwar spheres of influence over former Ottoman territories. Encyclopaedia Britannica.

At sea, the Royal Navy’s distant blockade constrained Central Powers’ trade, while Germany’s U‑boats waged commerce warfare; the largest fleet action, Jutland (1916), was tactically indecisive but left British naval dominance intact. Encyclopaedia Britannica; Encyclopaedia Britannica.

Technology and methods of warfare

The conflict accelerated adoption of machine guns, heavy artillery, barbed wire, aircraft, and chemical agents; Germany conducted the first large‑scale chlorine gas attack at Ypres on 22 April 1915. National WWI Museum and Memorial. Britain pioneered armored warfare, introducing tanks in combat at the Somme (Flers‑Courcelette) on 15 September 1916. Imperial War Museums. German unrestricted submarine warfare, resumed in February 1917, aimed to cut Britain’s lifelines but was increasingly blunted by the convoy system and anti‑submarine measures. Encyclopaedia Britannica; Imperial War Museums; Encyclopaedia Britannica.

United States entry

U.S. neutrality ended after Germany’s return to unrestricted submarine warfare and the disclosure of the Zimmermann Telegram, in which Berlin proposed a German–Mexican alliance against the United States. U.S. National Archives. Congress declared war on Germany on 6 April 1917; by late 1918 over two million American troops had deployed to France, contributing materially to the Allied Hundred Days Offensive. Office of the Historian, U.S. State Department; U.S. Senate, Historical Documents; Encyclopaedia Britannica.

Russia and the Eastern Front

The 1917 Russian Revolution precipitated Russia’s exit from the war; the Bolshevik government signed the Treaty of Brest‑Litovsk on 3 March 1918, ceding vast territories and freeing German forces for a western offensive. Encyclopaedia Britannica; Encyclopaedia Britannica.

Armistices and collapse of the Central Powers

In autumn 1918, the Central Powers unraveled: Bulgaria signed an armistice on 29 September, the Ottoman Empire on 30 October, and Austria‑Hungary on 3 November. Encyclopaedia Britannica, Key Facts. Germany, exhausted after the failed 1918 Spring Offensives and the Allied Hundred Days Offensive, sought terms; the Armistice of 11 November 1918 ended fighting on the Western Front at 11:00 a.m. CET. Encyclopaedia Britannica; Encyclopaedia Britannica.

Peace settlements and international order

The Paris Peace Conference (1919–1920) produced a series of treaties, most notably the Treaty of Versailles (28 June 1919), which imposed territorial losses, military restrictions, and reparations on Germany and incorporated the League of Nations Covenant. Encyclopaedia Britannica; Avalon Project, Yale Law School. The League’s Covenant (Part I of Versailles) established collective security mechanisms and a mandate system over former German and Ottoman territories. United Nations Office at Geneva; Encyclopaedia Britannica. The war precipitated the collapse of the German, Austro‑Hungarian, Ottoman, and Russian empires and redrew borders across Europe and the Middle East. Encyclopaedia Britannica; Encyclopaedia Britannica.

Home fronts, society, and the human toll

Mobilization blurred lines between civilian and military spheres, with state‑directed economies, rationing, and mass propaganda characteristic of “total war.” [David Stevenson, 1914–1918](book://David Stevenson|1914–1918: The History of the First World War|Penguin|2005). Women entered war industries, transport, agriculture, and auxiliary services in unprecedented numbers; in Britain alone about 900,000 women worked in munitions amid hazardous conditions. Imperial War Museums. Overall casualties reached about 40 million, including roughly 9–11 million military dead and 6–13 million civilian dead, with additional mortality from malnutrition and disease. Encyclopaedia Britannica, Key Facts; Encyclopaedia Britannica.

The 1918–1919 influenza pandemic overlapped the war’s final year, infecting an estimated one‑third of the world’s population and causing at least 50 million deaths globally; wartime troop movements and crowding facilitated spread. U.S. Centers for Disease Control and Prevention; Encyclopaedia Britannica; CDC Timeline.

Selected topics and terms

• –Zimmermann Telegram (1917): German proposal of a German–Mexican alliance against the U.S., whose disclosure helped shift American opinion toward war. U.S. National Archives.
• –Battle of the Somme (1916): Allied offensive marked by unprecedented casualties and the first battlefield use of tanks. Encyclopaedia Britannica; Imperial War Museums.
• –Gallipoli Campaign (1915–1916): Allied attempt to open a sea route to Russia via the Dardanelles; strategic failure with long‑term political repercussions. Encyclopaedia Britannica.
• –Treaty of Versailles (1919): Peace with Germany, including reparations and the League Covenant. Avalon Project; Encyclopaedia Britannica.
• –League of Nations (1920–1946): International organization born from the peace conference to promote collective security and international cooperation. United Nations Office at Geneva; Encyclopaedia Britannica.
• –Russian Revolution (1917): Overthrow of the imperial regime and rise of the Bolsheviks, leading to the Treaty of Brest‑Litovsk. Encyclopaedia Britannica.