World War I: The United Kingdom declares war on Austria-Hungary; the countries of the British Empire follow suit.
Austria-Hungary, often referred to as the Austro-Hungarian Empire or the Dual Monarchy, was a constitutional monarchy and great power in Central Europe between 1867 and 1918. It was formed with the Austro-Hungarian Compromise of 1867 in the aftermath of the Austro-Prussian War and was dissolved shortly after its defeat in the First World War.
Austria-Hungary was ruled by the House of Habsburg and constituted the last phase in the constitutional evolution of the Habsburg monarchy. It was a multinational state and one of Europe's major powers at the time. Austria-Hungary was geographically the second-largest country in Europe after the Russian Empire, at 621,538 km2 (239,977 sq mi) and the third-most populous (after Russia and the German Empire). The Empire built up the fourth-largest machine building industry in the world, after the United States, Germany, and the United Kingdom. Austria-Hungary also became the world's third-largest manufacturer and exporter of electric home appliances, electric industrial appliances, and power generation apparatus for power plants, after the United States and the German Empire.At its core was the dual monarchy which was a real union between Cisleithania, the northern and western parts of the former Austrian Empire, and the Kingdom of Hungary. Following the 1867 reforms, the Austrian and Hungarian states were co-equal in power. The two states conducted common foreign, defense, and financial policies, but all other governmental faculties were divided among respective states. A third component of the union was the Kingdom of Croatia-Slavonia, an autonomous region under the Hungarian crown, which negotiated the CroatianHungarian Settlement in 1868. After 1878, Bosnia and Herzegovina came under Austro-Hungarian joint military and civilian rule until it was fully annexed in 1908, provoking the Bosnian crisis among the other powers.Austria-Hungary was one of the Central Powers in World War I, which began with an Austro-Hungarian war declaration on the Kingdom of Serbia on 28 July 1914. It was already effectively dissolved by the time the military authorities signed the armistice of Villa Giusti on 3 November 1918. The Kingdom of Hungary and the First Austrian Republic were treated as its successors de jure, whereas the independence of the West Slavs and South Slavs of the Empire as the First Czechoslovak Republic, the Second Polish Republic, and the Kingdom of Yugoslavia, respectively, and most of the territorial demands of the Kingdom of Romania and the Kingdom of Italy were also recognized by the victorious powers in 1920.
World War I, often abbreviated as WWI or WW1, also known as the First World War and contemporaneously known as the Great War and by other names, was an international conflict that began on 28 July 1914 and ended on 11 November 1918. 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. 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. 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.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. 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. Austria-Hungary blamed Serbia and the interlocking alliances involved the Powers in a series of diplomatic exchanges known as the July Crisis. On 28 July, Austria-Hungary declared war on Serbia; Russia came to Serbia's defence and by 4 August, the conflict had expanded to include Germany, France and Britain, along with their respective colonial empires. 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.
Facing 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. 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. By contrast, the Eastern Front was far more fluid, with Austria-Hungary and Russia gaining, then losing large swathes of territory. Other significant theatres included the Middle East, the Alpine Front and the Balkans, bringing Bulgaria, Romania and Greece into the war.
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. In Russia, the Bolsheviks seized power in the 1917 October Revolution and made peace in the March 1918 Treaty of Brest-Litovsk, freeing up large numbers of German troops. By transferring these to the Western Front, the German General Staff hoped to win a decisive victory before American reinforcements could impact the war, and launched the March 1918 German spring offensive. Despite initial success, it was soon halted by heavy casualties and ferocious defence; in August, the Allies launched the Hundred Days Offensive and although the German army continued to fight hard, it could no longer halt their advance.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 revolution at home and an army on the verge of mutiny, Kaiser Wilhelm abdicated on 9 November and the new German government signed the Armistice of 11 November 1918, bringing the fighting to a close. The 1919 Paris Peace Conference imposed various settlements on the defeated powers, the best known being the Treaty of Versailles. The dissolution of the Russian, German, Ottoman and Austro-Hungarian empires 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 1939.