Why Did France Enter World War 1?

As a result of harsh treatment from Germany following the Prussian War in 1871, France had been extremely hostile toward Germany. France had to cede to Germany most of Alsace and Lorraine and pay significant reparations. When Germany declared war on Russia in 1914 during WWI, France was obligated under terms of a treaty with Russia to assist Russia. As such, France entered the War. In addition, France was facing the advancement of over 3 million German troops to France.