The assassination of Archduke Franz Ferdinand and his wife by a member of the Young Bosnian revolutionary movement is generally seen as the single event that formally began World War I. This led directly to conflict between Austria-Hungary and Serbia.
Each of these countries had a web of alliances with other major powers of the time, who also had their own strategic interests for supporting one side or another. After Austria-Hungary declared war on Serbia, Russia's mobilization in response led to a declaration of war on them by Germany. In turn, this eventually led to France, Britain and Japan becoming involved.