Answer :
The assassination of Archduke Franz Ferdinand in June 1914 by a Serbian nationalist outraged Austria-Hungary. Previous to the assassination, many European contries had been forming alliancies which now gained great importance. Austria-Hungary had made an alliance with Germany in 1879. Italy then joined this alliance in 1882. This alliance was against Russia, who also signed alliances with France in 1894. Britain then joined this alliance in 1907 to protect themselves against Germany's growing threat. Russia had also made an alliance with Serbia before the imminent threat of war because of the large number of Slavs that populated Serbia whom Russia swore to protect.
As Austria-Hungary declared war on Serbia on July 28, the rest of the countries had to follow suit. Supporting Austria-Hungary, Germany and Italy entered the conflict and Russia, France and Britain did so too.