One of the main causes of the war was rivalry between groups of European powers over trade. Countries formed defensive alliances, which meant that an attack on one country would automatically involve its allies. The 2 main alliances were ''Triple entente''(allied powers)-Britain France Russia and ''Triple alliance''(central powers)-Germany Austria-Hungary.

Austria knew that many of the Serbs in the south of its empire wanted to join Serbia and felt threatened when Serbia greatly increased its territory in the Balkan Wars. Austria wanted to fight and defeat Serbia and found an excuse when the Archduke of Austria was assassinated by a Serb nationalist. The 1^st World War had begun and continued from 1914 to 1918. At first the Central Powers were successful but when the United States became involved Central Powers retreated. 40 million lives were lost.