Russia was bound by treaty with Serbia, Montenegro declared war on Austria-Hungary. Germany declared war on Russia.
France bound by treaty with Russia declared war on Germany, Great Britain declared war on Germany after Germany refused to respect the neutrality and sovereignty of Belgium.
United states tried to stay neutral but were to threatned by Germany's submarine attacks on Trade vessels to Germany's enemies.
Japan had a military agreement with Britan, so declared war on Germany.
Serbia
After the Austro-Hungary monarchy declared war on Serbia, France and Russia who were protecting Serbia declared war on Austria-Hungary. and then Germany declared war on Russia and France, then UK declared war on Germany...
Austria-Hungary's future king was assassinated.
Russia
Austria Hungary declared war on Serbia.
Everything!!!
Serbia Serbia
Serbia was in WWI because Austria-Hungary declared war on them.
Answer this question… Russia had an alliance with Serbia, and Germany had an alliance with Austria-Hungary.
Russia.
Austria-Hungary, declared war on Serbia
Austria-Hungary declared war on Serbia at the start of World War I.