Black Hand
Austria-Hungary blamed Serbia for the assassination. Austria-Hungary then declared war on Serbia, and Russia joined the war on Serbia's side because the people of Serbia were Slavic, like Russia, and the Slavic countries had agreed to help each other if they were attacked. Since Russia is a large country it had to move soldiers closer to the war, but Germany feared that Russia's soldiers would also attack Germany. Russia did not like Germany because of things Germany had done in the past to become stronger. Germany declared war on Russia, and used a plan to attack Russia that had been created before the war, but it also involved attacking France and Belgium. France and Great Britain joined the war because Great Britain had agreed to help Belgium if it were ever attacked by someone else.