Russia joined the war on Serbia's side because the people of Serbia were Slavic just like Russia and the Slavic countries had agreed to help each other if they were attacked. Since the Russian Empire was 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 began to carry out a plan created long before to fight a war in Europe. Because Germany is in the middle of Europe, Germany could not attack to the east towards Russia without weakening itself in the west, towards France. Germany's plan involved quickly defeating France in the west before Russia was ready to fight, and then moving her armies to the east to face Russia. Germany could not quickly invade France directly, because France had put a lot of forts on the border, so Germany invaded the neighboring country of Belgium to then invade France through the undefended French/Belgian border. Great Britain then joined the war, saying they wanted to protect Belgium. Some historians think that even if Germany had stayed out of Belgium, the British would have still joined the war to help France.