World War I (also known as the First World War, Great War or War of Wars, abbreviated WWI) was a military conflict centered on Europe that began in the summer of 1914. The fighting ended in late 1918. This conflict involved all of the world’s great powers, assembled in two opposing alliances: the Allies (centred around the Triple Entente) and the Central Powers. More than 70 million military personnel, including 60 million Europeans, were mobilized in one of the largest wars in history. More than 9 million combatants were killed, due largely to great technological advances in firepower without corresponding ones in mobility. It was the second deadliest conflict in history.
The term World War One is particularly common in American English, whereas in Britain and the The Commonwealth, it is more commonly called the First World War. This term was first coined in 1920 as the title of Charles à Court Repington‘s book, but references to it being the first war did not become popular until World War II. The terms World War One and Two were first used in Time magazine in 1938. During and in the aftermath of the conflict it was called the Great War, particularly in British newspapers, whereas US media preferred simply the World War. It was also known as the War to End All Wars.
The assassination on 28 June 1914 of Archduke Franz Ferdinand of Austria, the heir to the throne of Austria-Hungary, is seen as the immediate trigger of the war. Long-term causes, such as imperialistic foreign policies of the great powers of Europe, such as the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, the Russian Empire, the British Empire, France, and Italy, played a major role. Ferdinand’s assassination by a Yugoslav nationalist resulted in a Habsburg ultimatum against the Kingdom of Serbia. Several alliances formed over the past decades were invoked, so within weeks the major powers were at war; as all had colonies, the conflict soon spread around the world.
On 28 July the conflict opened with the Austro-Hungarian invasion of Serbia, followed by the German invasion of Belgium, Luxembourg and France; and a Russian attack against Germany. After the German march on Paris was brought to a halt, the Western Front settled into a static battle of attrition with a trench line that changed little until 1917. In the East, the Russian army successfully fought against the Austro-Hungarian forces but was forced back by the German army. Additional fronts opened after the Ottoman Empire joined the war in 1914, Italy and Bulgaria in 1915 and Romania in 1916. The Russian Empire collapsed in 1917, and Russia left the war after the October Revolution later that year. After a 1918 German offensive along the western front, United States forces entered the trenches and the allies drove back the German armies in a series of successful offensives. Germany agreed to a cease fire on 11 November 1918, later known as Armistice Day.
By the war’s end, four major imperial powers—the German, Russian, Austro-Hungarian and Ottoman Empires—had been militarily and politically defeated. The last two ceased to exist. The revolutionized Soviet Union emerged from the Russian Empire, while the map of central Europe was completely redrawn into numerous smaller states. The League of Nations was formed in the hope of preventing another such conflict. The European nationalism spawned by the war and the break-up of empires, and the repercussions of Germany’s defeat and the Treaty of Versailles led to the beginning of World War II in 1939.
-from Wikipedia (http://en.wikipedia.org/wiki/World_War_I)