Answer :

Started with the assassination of Arch Duke Franz Ferdinand. 
The first world war began in August 1914. It was directly triggered by the assassination of the Austrian archduke, Franz Ferdinand and his wife, on 28th June 1914 by Bosnian revolutionary, Gavrilo Princip.

This event was, however, simply the trigger that set off declarations of war. The actual causes of the war are more complicated and are still debated by historians today.

Alliances
An alliance is an agreement made between two or more countries to give each other help if it is needed. When an alliance is signed, those countries become known as Allies.

A number of alliances had been signed by countries between the years 1879 and 1914. These were important because they meant that some countries had no option but to declare war if one of their allies. declared war first.
Imperialism
Imperialism is when a country takes over new lands or countries and makes them subject to their rule. By 1900 the British Empire extended over five continents and France had control of large areas of Africa. With the rise of industrialism countries needed new markets. The amount of lands 'owned' by Britain and France increased the rivalry with Germany who had entered the scramble to acquire colonies late and only had small areas of Africa.

Militarism
Militarism means that the army and military forces are given a high profile by the government. The growing European divide had led to an arms race between the main countries. The armies of both France and Germany had more than doubled between 1870 and 1914 and there was fierce competition between Britain and Germany for mastery of the seas. The British had introduced the 'Dreadnought', an effective battleship, in 1906. The Germans soon followed suit introducing their own battleships. The German, Von Schlieffen also drew up a plan of action that involved attacking France through Belgium if Russia made an attack on Germany.

Nationalism
Nationalism means being a strong supporter of the rights and interests of one's country. The Congress of Vienna, held after Napoleon's exile to Elba, aimed to sort out problems in Europe. Delegates from Britain, Austria, Prussia and Russia (the winning allies) decided upon a new Europe that left both Germany and Italy as divided states. Strong nationalist elements led to the re-unification of Italy in 1861 and Germany in 1871. The settlement at the end of the Franco-Prussian war left France angry at the loss of Alsace-Lorraine to Germany and keen to regain their lost territory. Large areas of both Austria-Hungary and Serbia were home to differing nationalist groups, all of whom wanted freedom from the states in which they lived.

Moroccan Crisis
In 1904 Morocco had been given to France by Britain, but the Moroccans wanted their independence. In 1905, Germany announced her support for Moroccan independence. War was narrowly avoided by a conference which allowed France to retain possession of Morocco. However, in 1911, the Germans were again protesting against French possession of Morocco. Britain supported France and Germany was persuaded to back down for part of French Congo.

Bosnian Crisis
In 1908, Austria-Hungary took over the former Turkish province of Bosnia. This angered Serbians who felt the province should be theirs. Serbia threatened Austria-Hungary with war, Russia, allied to Serbia, mobilised its forces. Germany, allied to Austria-Hungary mobilised its forces and prepared to threaten Russia. War was avoided when Russia backed down. There was, however, war in the Balkans between 1911 and 1912 when the Balkan states drove Turkey out of the area. The states then fought each other over which area should belong to which state. Austria-Hungary then intervened and forced Serbia to give up some of its acquisitions. Tension between Serbia and Austria-Hungary was high.