World War-I has its roots in the assassination of a prince. On 28 June 1914, Archduke Francis Ferdinand, the heir to the throne of Austria-Hungary was assassinated by a Serbian terrorist at Sarajevo, the capital of Bosnia. Austria saw the hands of Serbia behind this and served Serbia an ultimatum. Serbia refused to accept one of the demands of ultimatum. Hence, on 28 July 1914, Austria declared war on Serbia.  Then, Germany declared war on Russia and France. Britain declared war on Germany. Japan declared war on Germany with a view to capture its colonies in the Far East. Turkey and Bulgaria joined on the side of Germany. Italy initially remained neutral and later joined the war against Germany in 1915.

There are various reasons for the breakout of the war:

1. Imperialist Rivalries: The scramble for colonies led to emergence of conflict between imperialist powers. By the last decade of 19th century almost all areas were under imperialist control and further conquest could only happen by dispossessing some other country. Rivalries resulted in attempts to re-divide the world creating conditions of war.
   
   
2. Progress of the latecomer Germany: Germany made massive progress after its unification in 1870. It became leading producer of iron, steel and coal and left behind France and Britain. It entered shipping trade as well and possessed Imperator, the largest ship in the world. Since Germany was a late comer it could not grab as much colony as it desired.
   
   
3. Clash of interests: Both Italy and Austria had their ambitions in the Ottoman Empire. Japan fought with Russia for extending its territorial possessions in the Far East. There was an intense naval rivalry between Germany and Britain as Britain defended its large territory. Germans accused Britain, Russia and France of trying to 'encircle' it.
   
   
4. Serbian Nationalism: Serbia had the ambition of uniting all Slavs many of whom lived in Austria – Hungarian empire, which consisted of people from different nationalities (Slovaks, Czechs, Italian, etc.). Therefore, even Austria wanted to destroy Serbia.
   
   
5. Alliance Formation: Opposing groups were formed and vast sums of money were spent to increase size of army and navy and develop deadly weapons. Europe became a vast armed camp. Propaganda for war and projecting own country as superior to other started.
   
   
6. a) Triple Alliance (1882) - Germany, Austria-Hungary and Italy.
   
   
7. b) Triple Entente or Understanding (1907) - France, Britain and Russia. Loose group based on mutual understanding.
   
   

The First World War was the most frightful war the world had seen so far in terms of devastation it caused, the number of people who fought it, the famines and the social problems it created. Instead of destroying imperialism, it helped the victorious powers in enlarging their possessions.