The war was started by the assassination of the heir to the Austrian throne Franz Ferdinand in Sarajevo, capital of Bosnia, a province of Austria at the time.(June, 28, 1914) Serbia had always wanted to crush the Serbs in Bosnia and Serbia itself, who was helping Austrian- Serbs in an uprising against Austria. Since the Serbs had always fought the Austrian government and threatened their 'patchwork' quilt of nations, Austria used the assassination of Franz Ferdinand as an excuse to send an ultimatum to Serbia. Germany had already agreed to help Austria- Hungary in the event of war and had given Austria a blank cheque. At this time Russia, Germany, Serbia and Austria are prepared to fight. And with this Austria- Serbia conflict the World War 1 started and later grew with France, England, Belgium and many other nations.

The simplest answer is that the immediate cause was the assassination of Franz Ferdinand, the archduke of Austria-Hungary. His death at the hands of Gavrilo Princip – a Serbian nationalist with ties to the secretive military group known as the Black Hand – propelled the major European military powers towards war.

World War 1 was caused by a complex network of treaties and alliances. The immediate cause was the assassination of Archduke Franz Ferdinand and his wife Sophie of Hohenberg (28th June 1914), the objective of which was to break off the Serbian parts of the Austro-Hungarian Empire to create a wholly pan-slavic or Yugoslavian state.

Vienna could not allow this outrage (Franz Ferdinand was the heir apparent to the throne) but in moving against the Serbians Russian outrage was generated as Russia supported the Slavs. If Russia moved against Austria, Germany could either stand by and see its strongest ally severely weakened or destroyed or mobilise to help. However, if Germany mobilised against Russia, France was bound by treaty to help Russia (France still had bad memories of defeat by united German armies in the Franco-Prussian War 1870-71) and this would mean a counter-attack on Germany from the West, which is in fact what happened. The German response was to follow the von Schlieffen plan, which meant that troops would have to be moved by railway, and to do this meant invading and occupying parts of Belgium. By treaty (Treaty of London 1839),