Answer :
The explosive that was World War One had been long in the stockpiling; the spark was the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, in Sarajevo on 28 June 1914. (Click here to view film footage of Ferdinand arriving at Sarajevo's Town Hall on 28 June 1914.)
Ferdinand's death at the hands of the Black Hand, a Serbian nationalist secret society, set in train a mindlessly mechanical series of events that culminated in the world's first global war
Austria-Hungary, unsatisfied with Serbia's response to her ultimatum (which in the event was almost entirely placatory: however her jibbing over a couple of minor clauses gave Austria-Hungary her sought-after cue) declared war on Serbia on 28 July 1914.Russia, bound by treaty to Serbia, announced mobilisation of its vast army in her defence, a slow process that would take around six weeks to complete.Germany, allied to Austria-Hungary by treaty, viewed the Russian mobilisation as an act of war against Austria-Hungary, and after scant warning declared war on Russia on 1 August.France, bound by treaty to Russia, found itself at war against Germany and, by extension, on Austria-Hungary following a German declaration on 3 August. Germany was swift in invading neutral Belgium so as to reach Paris by the shortest possible route.Britain, allied to France by a more loosely worded treaty which placed a "moral obligation" upon her to defend France, declared war against Germany on 4 August. Her reason for entering the conflict lay in another direction: she was obligated to defend neutral Belgium by the terms of a 75-year old treaty. With Germany's invasion of Belgium on 4 August, and the Belgian King's appeal to Britain for assistance, Britain committed herself to Belgium's defence later that day. Like France, she was by extension also at war with Austria-Hungary.With Britain's entry into the war, her colonies and dominions abroad variously offered military and financial assistance, and included Australia, Canada, India, New Zealand and the Union of South Africa.United States President Woodrow Wilson declared a U.S. policy of absolute neutrality, an official stance that would last until 1917 when Germany's policy of unrestricted submarine warfare - which seriously threatened America's commercial shipping (which was in any event almost entirely directed towards the Allies led by Britain and France) - forced the U.S. to finally enter the war on 6 April 1917.Japan, honouring a military agreement with Britain, declared war on Germany on 23 August 1914. Two days later Austria-Hungary responded by declaring war on Japan.Italy, although allied to both Germany and Austria-Hungary, was able to avoid entering the fray by citing a clause enabling it to evade its obligations to both. In short, Italy was committed to defend Germany and Austria-Hungary only in the event of a 'defensive' war; arguing that their actions were 'offensive' she declared instead a policy of neutrality. The following year, in May 1915, she finally joined the conflict by siding with the Allies against her two former allies.