Genre: Information/Explanation

World War I , also called First World War or Great War , an international conflict that in 1914 – 18 embroiled most of the nations of Europe along with Russia , the United States , the Middle East , and other regions . The war pitted the Central Powers – mainly Germany , Austria-Hungary , and Turkey – against the Allies – mainly France , Great Britain , Russia , Italy , Japan , and , from 1917 , the United States . It ended with the defeat of the Central Powers . The war was virtually unprecedented in the slaughter , carnage , and destruction it caused . A British soldier inside a trench on the Western Front during World War I , 1914 – 18 . Hulton Archive / Getty Images World War I was one of the great watersheds of 20th-century geopolitical history . It led to the fall of four great imperial dynasties ( in Germany , Russia , Austria-Hungary , and Turkey ) , resulted in the Bolshevik Revolution in Russia , and , in its destabilization of European society , laid the groundwork for World War II . A collection of significant facts about World War I. The outbreak of war With Serbia already much aggrandized by the two Balkan Wars ( 1912 – 13 , 1913 ) , Serbian nationalists turned their attention back to the idea of " liberating " the South Slavs of Austria-Hungary . Colonel Dragutin Dimitrijević , head of Serbia's military intelligence , was also , under the alias " Apis , " head of the secret society Union or Death , pledged to the pursuit of this pan-Serbian ambition . Believing that the Serbs' cause would be served by the death of the Austrian archduke Franz Ferdinand , heir presumptive to the Austrian emperor Franz Joseph , and learning that the Archduke was about to visit Bosnia on a tour of military inspection , Apis plotted his assassination . Nikola Pašić , the Serbian prime minister and an enemy of Apis , heard of the plot and warned the Austrian government of it , but his message was too cautiously worded to be understood . Overview of the start of World War I , including details of the June 28 , 1914 , assassination of ... Europeans were fighting heavily on two fronts before the U.S. entered the war in 1917 . At 11 : 15 am on June 28 , 1914 , in the Bosnian capital , Sarajevo , Franz Ferdinand and his morganatic wife , Sophie , duchess of Hohenberg , were shot dead by a Bosnian Serb , Gavrilo Princip . The chief of the Austro-Hungarian general staff , Franz , Graf ( count ) Conrad von Hötzendorf , and the foreign minister , Leopold , Graf von Berchtold , saw the crime as the occasion for measures to humiliate Serbia and so to enhance Austria-Hungary's prestige in the Balkans . Conrad had already ( October 1913 ) been assured by William II of Germany ' s support if Austria-Hungary should start a preventive war against Serbia . This assurance was confirmed in the week following the assassination , before William , on July 6 , set off upon his annual cruise to the North Cape , off Norway . Austrian archduke Franz Ferdinand and his wife , Sophie , riding in an open carriage at Sarajevo ... Henry Guttmann – Hulton Archive / Getty Images The Austrians decided to present an unacceptable ultimatum to Serbia and then to declare war , relying on Germany to deter Russia from intervention . Though the terms of the ultimatum were finally approved on July 19 , its delivery was postponed to the evening of July 23 , since by that time the French president , Raymond Poincaré , and his premier , René Viviani , who had set off on a state visit to Russia on July 15 , would be on their way home and therefore unable to concert an immediate reaction with their Russian allies . When the delivery was announced , on July 24 , Russia declared that Austria-Hungary must not be allowed to crush Serbia . Serbia replied to the ultimatum on July 25 , accepting most of its demands but protesting against two of them – namely , that Serbian officials ( unnamed ) should be dismissed at Austria-Hungary's behest and that Austro-Hungarian officials should take part , on Serbian soil , in proceedings against organizations hostile to Austria-Hungary . Though Serbia offered to submit the issue to international arbitration , Austria-Hungary promptly severed diplomatic relations and ordered partial mobilization . Home from his cruise on July 27 , William learned on July 28 how Serbia had replied to the ultimatum . At once he instructed the German Foreign Office to tell Austria-Hungary that there was no longer any justification for war and that it should content itself with a temporary occupation of Belgrade . But , meanwhile , the German Foreign Office had been giving such encouragement to Berchtold that already on July 27 he had persuaded Franz Joseph to authorize war against Serbia . War was in fact declared on July 28 , and Austro-Hungarian artillery began to bombard Belgrade the next day . Russia then ordered partial mobilization against Austria-Hungary , and on July 30 , when Austria-Hungary was riposting conventionally with an order of mobilization on its Russian frontier , Russia ordered general mobilization . Germany , which since July 28 had still been hoping , in disregard of earlier warning hints from Great Britain , that Austria-Hungary's war against Serbia could be " localized " to the Balkans , was now disillusioned insofar as eastern Europe was concerned . On July 31 Germany sent a 24-hour ultimatum requiring Russia to halt its mobilization and an 18-hour ultimatum requiring France to promise neutrality in the event of war between Russia and Germany . Both Russia and France predictably ignored these demands . On August 1 Germany ordered general mobilization and declared war against Russia , and France likewise ordered general mobilization . The next day Germany sent troops into Luxembourg and demanded from Belgium free passage for German troops across its neutral territory . On August 3 Germany declared war against France . In the night of August 3 – 4 German forces invaded Belgium . Thereupon , Great Britain , which had no concern with Serbia and no express obligation to fight either for Russia or for France but was expressly committed to defend Belgium , on August 4 declared war against Germany . Austria-Hungary declared war against Russia on August 5 ; Serbia against Germany on August 6 ; Montenegro against Austria-Hungary on August 7 and against Germany on August 12 ; France and Great Britain against Austria-Hungary on August 10 and on August 12 , respectively ; Japan against Germany on August 23 ; Austria-Hungary against Japan on August 25 and against Belgium on August 28 . Romania had renewed its secret anti-Russian alliance of 1883 with the Central Powers on February 26 , 1914 , but now chose to remain neutral . Italy had confirmed the Triple Alliance on December 7 , 1912 , but could now propound formal arguments for disregarding it : first , Italy was not obliged to support its allies in a war of aggression ; second , the original treaty of 1882 had stated expressly that the alliance was not against England . On September 5 , 1914 , Russia , France , and Great Britain concluded the Treaty of London , each promising not to make a separate peace with the Central Powers . Thenceforth , they could be called the Allied , or Entente , powers , or simply the Allies . The outbreak of war in August 1914 was generally greeted with confidence and jubilation by the peoples of Europe , among whom it inspired a wave of patriotic feeling and celebration . Few people imagined how long or how disastrous a war between the great nations of Europe could be , and most believed that their country's side would be victorious within a matter of months . The war was welcomed either patriotically , as a defensive one imposed by national necessity , or idealistically , as one for upholding right against might , the sanctity of treaties , and international morality . Britannica Web sites World War I lasted from 1914 to 1918 . It was known at first as the Great War and the War to End All Wars . It was the largest war that the world had seen up to that time . Most of the battles took place in Europe and the Middle East . More than 8 million soldiers and sailors died , and more than 20 million were injured . The war was so bad that many people thought no one would ever start another war . However , the results of World War I helped to cause World War II , which broke out in 1939 . A major international conflict fought from 1914 to 1918 , World War I was the most deadly and destructive war the world had ever seen to that time . More than 25 countries eventually participated , aligned with either the Allied or the Central powers . The Allies-who won the war-included primarily France , Great Britain , Russia , Italy , Japan , and , from 1917 , the United States . The Central Powers consisted mainly of Germany , Austria-Hungary , and the Ottoman Empire ( Turkey ) . World War I felled four great imperial dynasties , in Germany , Russia , Austria-Hungary , and Turkey . It led to revolution in Russia , destabilized Europe , and laid the foundations for World War II . ( For a chronology of events for World War I , see World War I Chronology . )