The First World War was the first truly global conflict. From 1914 to 1918, fighting took place across several continents, at sea and, for the first time, in the air. This was war on an unprecedented scale, with battles often lasting months instead of days.
World War I (28 July 1914 – 11 November 1918), often abbreviated as WWI, also known as the Great War, was one of the deadliest global conflicts in history. The main belligerents included much of Europe and their colonial empires, the Russian Empire, the United States, the Ottoman Empire and the Japanese Empire, with fighting occurring throughout Europe, the Middle East, Africa, the Pacific, and parts of Asia. An estimated 9 million soldiers were killed in combat, plus another 23 million wounded, while 5 million civilians died as a result of military action, hunger, and disease. Millions more died in genocides within the Ottoman Empire and in the 1918 influenza pandemic, which was exacerbated by the movement of combatants during the war
Prior to 1914, the European great powers were divided between the Triple Entente (comprising France, Russia, and Britain) and the Triple Alliance (containing Germany, Austria-Hungary, and Italy). Tensions in the Balkans came to a head on 28 June 1914, following the assassination of Archduke Franz Ferdinand of Austria-Hungary by Gavrilo Princip, a Bosnian Serb. Austria-Hungary blamed Serbia, which led to the July Crisis, an unsuccessful attempt to avoid conflict through diplomacy. On 28 July 1914, Austria-Hungary declared war on Serbia, while Russia came to the latter’s defence. By the 4th of August, Germany, France, and Britain (along with their respective colonies) were also drawn into the war. In November 1914, the Ottoman Empire, Germany, and Austria-Hungary formed the Central Powers, and on 26 April 1915, Italy joined Britain, France, Russia, and Serbia as the Allies of World War I.
The German strategy in 1914 was to concentrate its forces on defeating France in six weeks, before moving them to the Eastern Front and doing the same to Russia. However, this was defeated at the Marne in September 1914, and the year ended with the two sides facing each other along the Western Front, a continuous series of trenches stretching from the English Channel to Switzerland. The frontlines in the West changed little until 1917, while the Eastern Front was far more fluid, with both Austria-Hungary and Russia gaining and losing large swathes of territory. Other significant theatres included the Middle East, Italy, Asia Pacific, and the Balkans, which drew Bulgaria, Romania, and Greece into the war. Both Russia and Austria-Hungary suffered enormous casualties in the East throughout 1915, while Allied offensives in Gallipoli and the Western Front ended in failure. In 1916, German attacks at Verdun, and a Franco-British offensive on the Somme, led to heavy losses for limited strategic gains, while the Russian Brusilov offensive ground to a halt after early success. By 1917, Russia was on the verge of revolution, the French Nivelle offensive ended in failure, and British, French and German forces experienced heavy losses in Ypres, leaving all belligerents short of manpower and under severe economic stress. Shortages caused by the Allied naval blockade led Germany to initiate unrestricted submarine warfare, bringing the previously-neutral United States into the war on 6 April 1917.
In Russia, the Bolsheviks seized power in the 1917 October Revolution and exited the war with the March 1918 Treaty of Brest-Litovsk, freeing up large numbers of German troops. Germany used these additional resources to launch the March 1918 offensive, but was halted by stubborn Allied defence, heavy casualties, and supply shortages. When the Allies began the Hundred Days Offensive in August, the Imperial German Army continued to fight hard but could only slow the advance, not stop it. Towards the end of 1918, the Central Powers began to collapse; Bulgaria signed an armistice on 29 September, followed by the Ottomans on 31 October, then Austria-Hungary on 3 November. Isolated, facing the German Revolution at home and a military on the verge of mutiny, Kaiser Wilhelm II abdicated on 9 November, and the new German government signed the Armistice of 11 November 1918, bringing the conflict to a close. The Paris Peace Conference of 1919–1920 imposed various settlements on the defeated powers, with the best-known of these being the Treaty of Versailles. The dissolution of the Russian Empire in 1917, the German Empire in 1918, the Austria-Hungarian Empire in 1920, and the Ottoman Empire in 1922, led to numerous uprisings and the creation of independent states, including Poland, Czechoslovakia, and Yugoslavia. For reasons that are still debated, failure to manage the instability that resulted from this upheaval during the interwar period ended with the outbreak of World War II in September 1939.