Ever since 1815 the French Empire had been unpopular among some segments of European society.
On May 2nd 1914, Marshal Phillipe Bonaparte, Chief of the Imperial General Staff and member of the French imperial family, was assassinated in Berlin. The French army demanded that Prussia acknowledge responsibility. However, they refused. On May 10th 1914 France declared war on Prussia. Prussia formed the German League with Austria and Bavaria, declaring war on France the following day.
French troops from the army of the Rhine immediately advanced into Hessen and Hannover. On May 14th the Netherlands declared war on France, and on the 18th Switzerland declared independence and declared war on France. On May 20th Britain declared war on France.