Everything!!!
Serbia
Austria-Hungary's future king was assassinated.
Russia
Austria Hungary declared war on Serbia.
Serbia was in WWI because Austria-Hungary declared war on them.
Serbia Serbia
Answer this question… Russia had an alliance with Serbia, and Germany had an alliance with Austria-Hungary.
Austria-Hungary, declared war on Serbia
Austria-Hungary declared war on Serbia at the start of World War I.
Austria-Hungary, with the death of Archduke Ferdinand, declared war on Serbia, which caused a chain reaction of war induced countries. Austria Hungary to Serbia, then Russia to Germany, United Kingdom and so on.
Austria-Hungary declared war on serbia and Germany joined because it was allied with austria.