It was a Serb who assassinated the Hungarian Archduke Franz Ferdinand.
The 1st country to declare war on Serbia was Austria-Hungary.
Serbia was in WWI because Austria-Hungary declared war on them.
Serbia Serbia
Austria-hungary attacked Serbia
Austria-Hungary declared war on Serbia at the start of World War I.
Russia
World War One was started when Serbia killed Archduke Francis Ferdidnand and his wife Sophia of Austria-Hungary. Serbia and Austria-Hungary were already at odds for a while but when the Archduke was killed Austria-Hungary called a state of war on Serbia.
The assassination of Archduke Franz Ferdinand of Austria in Sarajevo led Austria-Hungary to declare was on Serbia. This was to effectively be the beginning of World War I.
Serbia
it caused Serbia to declare war on Austria Hungary
it caused Serbia to declare war on austria hungary
It was a World War because a man killed the archduke of Austria-Hungary in Sebia. Austria-Hungary declared war on Serbia. Serbia had an alliance with Russia and then Russia joined the war. Russia was in the Triple Entante and pulled Britain and France into the war. Austria-Hungary had alliances with Germany and the Ottoman Empire. They also joined the war. That is why World War I was a world war.