First World War

Background

The Sarajevo assassination was the immediate trigger, against a background of imperial rivalry, armaments, Balkan nationalism and alliances. During the July Crisis, Austria-Hungary's ultimatum to Serbia, Russian mobilization, German declarations of war and invasion of Belgium, and British intervention widened a regional conflict.

Allies: the initial participants

France, the Russian Empire and the British Empire formed the core. Participants entered at different times and pursued different aims.

• Serbia and Montenegro — 1914: Fought Austria-Hungary in the Balkans. After the 1915 invasion, Serbian forces retreated, reorganized and continued fighting on the Salonika front.
• France — August 1914: A principal Western Front belligerent resisting German invasion, drawing on metropolitan and colonial troops.
• Russian Empire — August 1914: Fought Germany and Austria-Hungary in the east and the Ottomans in the Caucasus. Following the revolutions, Soviet Russia withdrew in March 1918.
• British Empire — 4 August 1914: Declared war on Germany; its role combined land warfare, naval blockade, shipping and imperial mobilization.
• Belgium — August 1914: Entered after Germany invaded its neutral territory; much of the country was occupied.
• Japan — 23 August 1914: Declared war on Germany and attacked Qingdao and German Pacific possessions while expanding its regional influence.

Allies: later entrants and their roles

• Italy — 23 May 1915: Declared war on Austria-Hungary. Despite its pre-war Triple Alliance membership, it remained neutral in 1914 and then joined the Allies. War with Germany followed in 1916.
• Portugal — 9 March 1916: Germany declared war after Portugal seized German ships. Portugal fought in Africa and sent troops to the Western Front.
• Romania — 27 August 1916: Declared war on Austria-Hungary and advanced into Transylvania before Central Powers counterattacks.
• United States — 6 April 1917: Declared war on Germany following renewed unrestricted submarine warfare and other disputes. It fought as an associated power rather than joining the Allies' formal alliance treaty.
• Greece — June 1917: Joined after a domestic split over neutrality and intervention; it fought on the Salonika front.
• Siam (now Thailand) — 22 July 1917: Declared war on Germany and Austria-Hungary, sending an expedition to France the following year and seeking revision of unequal treaties.
• Republic of China — 14 August 1917: Declared war on Germany and Austria-Hungary. Chinese labourers' transport, construction and logistical work was a major contribution; recruitment preceded formal entry.
• Brazil — 26 October 1917: Declared war on Germany and contributed naval operations and medical assistance.

Additional declarations of war

Cuba, Panama and Liberia declared war in 1917; Guatemala, Nicaragua, Costa Rica, Haiti and Honduras followed in 1918. Their contributions differed from those of powers fielding mass armies. Breaking diplomatic relations, declaring war and deploying combat troops were distinct forms of involvement.

Central Powers: four states

• German Empire — August 1914: Fought on both Western and Eastern Fronts, employing major land offensives and submarine warfare.
• Austria-Hungary — 28 July 1914: Opened the war by declaring war on Serbia and fought in the Balkans, eastern Europe and Italy. Austria and Hungary constituted one Dual Monarchy, not two independent belligerents.
• Ottoman Empire — October–November 1914: Attacked Russian Black Sea ports on 29 October, followed by war with Russia, Britain and France. Major fronts included the Caucasus, Gallipoli, Mesopotamia and Palestine.
• Bulgaria — 14 October 1915: Declared war on Serbia, fought in the Balkans and concluded an armistice in September 1918.

Dominions and colonial mobilization

Britain's entry involved its empire. Canada, Australia, New Zealand and the Union of South Africa contributed as dominions. Newfoundland was a separate dominion, not yet part of Canada. British India supplied soldiers and labourers for multiple theatres. Their wartime status should not be equated with present-day sovereignty.

France also mobilized colonial soldiers and workers from Africa and Indochina. Military campaigns depended on colonial labour, transport and resources as well as combat service.

Fronts and total war

Western Front trenches coexisted with mobile campaigns in the east, Balkan, Italian and Ottoman fronts, and colonial warfare in Africa and the Pacific. Blockades and submarines disrupted trade and civilian life. Prolonged mobilization expanded state control over production, food and labour.

The map highlights major European and Middle Eastern theatres. Arrows simplify selected advances from different dates through representative waypoints; they are not exact unit tracks, front lines or borders. Map numbers match the timeline.

Russian revolutions and the settlement

The February and October Revolutions occurred during the war; Soviet Russia withdrew through Brest-Litovsk. The collapse of Russian tsarism and the defeat and dissolution of the German, Austro-Hungarian and Ottoman empires transformed Europe and the Middle East. The 11 November 1918 armistice ended Western Front fighting; peace treaties, including Versailles in 1919, followed. Dates below use the Gregorian calendar.