Alliances were the main cause of World War I because it had split Europe's military forces into two camps: the Triple Alliance and the Triple Entente.
World war I was the battle between the central power and the allies. The central powers includes the countries Germany, Austria-Hungary, Bulgaria and the Ottoman Empire and allies included the countries like Serbia, Russia, France, Britain, Italy and the United States. it started in 1914 and ended in 1918.
The alliance is considered as the major cause of the world war as Russia formed the alliance with Serbia. Thus Germany declared the war with Russia as it had the alliance with Austria-Hungary.
Thus they formed the groups such as triple alliance which had the group of Germany, Austria-Hungary and their allies and second group named The Triple Entente which included Great Britain, France and Russia.
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