During the beginning of the 20th Century there were two main power blocks in Europe, Germany and Austria-Hungary, and the Triple Entente – Britain, France and Russia.
The Ottoman Empire or the Sick man of Europe as it was then often referred to was on the decline, it had lost most of its territories, at its peak it included the Balkans, however Greece declared its freedom in 1829 and in 1878 after a war fought between the Russians and the Turks – Montenegro, Serbia, Romania and Bulgaria were given their independence. Serbia wanted to annex Bosnia but it was placed under Austrian-Hungarian control, although it technically was still part of the Ottoman Empire – Until on July 24th 1908 Revolutionary officers called The Young Turks led a coup against the aged and corrupt Sultan Abdul Hamid II of the Ottoman Empire, Austria-Hungary took advantage of this by annexing Bosnia into the Habsburg Empire. Serbia looked to Russia for support but Russia was still recovering from their defeat against Japan in 1905, also Germany we’re prepared to back their ally Austria-Hungary at all costs, and Russia was not looking for another military conflict. Serbian nationalist decided to take matters into their own hands. Dragutin Dimitrijević formed a Secret society named the Black Hand which was dedicated to bring all Southern Slavic territories together. In 1903 he organised the assassination of pro Austrian ruler of Serbia King Alexander I and in 1911 he led an unsuccessful attempt on Austrian Emperor Franz Josef. Soon Dimitrijević set his eyes on the heir to the Austria-Hungary throne Archduke Franz Ferdinand, who once in power planned to stabilise the Austrian-Hungary Empire, which would make Dimitrijević’s plans extremely difficult.
On June 28th 1914 the Archduke Franz Ferdinand is in the back seat of an open top car with his wife during a procession in Sarajevo, Bosnia. A young Bosnian Serb by the name of Gavrilo Princip fired 2 shots murdering them both. After an investigation there was no evidence of direct Serbian involvement in any plot. Austria-Hungary went straight to their powerful allies Germany and was reassured, if Russia mobilised any troops, Germany would support Austria-Hungary. This has been referred to as a blank cheque. Austria-Hungary then sent an ultimatum to Serbia which was so extreme they knew it would be rejected, giving them a reason to go to war. And of course Serbia rejected it. On July 28th 1914 Austria-Hungary declared war on Serbia and bombed Belgrade.
Then the military alliances came into play.
30th July 1914– Russia declared war on Austria-Hungary.
1st August 1914 – Germany declared war on Russia. France mobilised its forces.
3rd August 1914 – Germany declared war on France and invaded via neutral Belgium.
4th August 1914– Great Britain declared war on Germany.
6th April 1917 – The United States of America declared war on Germany. click here to read why.
Soon all the major powers in Europe were at war simply because one man shot another.
Source – Infamous Assassinations, The Assassination of Archduke Franz Ferdinand