Shortages caused by the Allied naval blockade led Germany to initiate unrestricted submarine warfare in early 1917, bringing the previously neutral United States into the war on 6 April 1917. Other significant theatres included the Middle East, the Alpine Front and the Balkans, bringing Bulgaria, Romania and Greece into the war. By contrast, the Eastern Front was far more fluid, with Austria-Hungary and Russia gaining, then losing large swathes of territory.
This failed when their advance into France was halted at the Marne by the end of 1914, the two sides faced each other along the Western Front, a continuous series of trench lines stretching from the Channel to Switzerland that changed little until 1917. Bulgaria would join the Central Powers on 14 October 1915 to retake the territory lost to Serbia in the Second Balkan War and Romania join the Allied Powers on 27 August 1916 to take Transylvania, Bukovina and Banat, regions with an ethnic Romanian majority under the Austro-Hungarian Empire.įacing a war on two fronts, German strategy in 1914 was to defeat France, then shift its forces to the East and knock out Russia, commonly known as the Schlieffen Plan. In November, the Ottoman Empire, Germany and Austria formed the Central Powers, while in April 1915, Italy joined Britain, France, Russia and Serbia as the Allied Powers. Germany invaded France through Belgium which had a British guarantee of independence, while the Ottoman Empire attacked Russia wanting to regain its Caucasus possessions. On 28 July, Austria-Hungary declared war on Serbia Russia came to Serbia's defence and by 4 August, Germany came to Austria-Hungary's defense and France executed the Franco-Russian Alliance against Germany. Austria-Hungary blamed Serbia and the interlocking alliances involved the Powers in a series of diplomatic exchanges known as the July Crisis. Tensions in the Balkans came to a head on 28 June 1914 following the assassination of Archduke Franz Ferdinand, the Austro-Hungarian heir, by Gavrilo Princip, a Bosnian Serb. In 1914, the Great Powers were divided into two opposing alliances, the Triple Entente, consisting of France, Russia, and Britain, and the Triple Alliance, made up of Germany, Austria-Hungary, and Italy. The genocides perpetrated by the Ottomans and the 1918 Spanish flu pandemic spread by the movement of combatants during the war caused many millions of additional deaths worldwide. One of the deadliest conflicts in history, an estimated 9 million were killed in combat, while over 5 million civilians died from occupation, bombardment, hunger or disease. It involved much of Europe, as well as Russia, the United States and Turkey, and was also fought in the Middle East, Africa and parts of Asia.
World War I, often abbreviated as WWI or WW1, also known as the First World War or the Great War, was a global war that began on 28 July 1914 and ended on 11 November 1918. World War I: Mobilized forces per total population (in %)