What Was The Main Cause Of World War One?

The actual trigger to the war was the 28 June 1914 assassination in Serbia, by a Serb, of the heir to the Austro-Hungarian empire, Archduke Franz Ferdinand.  However Europe's major powers had long been building up to a major conflict.

The aggressive approach in the preceding years of Germany's ruler Kaiser Wilhelm II had put other nations on edge.  Deciding against renewing a treaty with Russia in favour, effectively, of an Austrian alliance helped drive Germany's neighbours, France and Russia, into an alliance of their own in 1894.  And when in 1898 Germany began building up its navy, the world's leading sea power Britain was alarmed enough to form alliances with France and Russia.  These alliances sprung into deadly reality very quickly after the 1914 assassination: Austria-Hungary retaliated with action against Serbia, whom they suspected of supporting the assassination.  Germany then offered unconditional support to the Austrians and Russia supported Serbia.  This brought in Russia's allies France and then Britain and the battle lines for a now inevitable war were thus drawn.

Some historians believe Germany had actually been planning a war for months, possibly with a long term aim of grabbing a colonial empire from their rivals for themselves.  

The assassination of someone-i cant remember who though