Four days after Austria-Hungary declared war on Serbia, Germany and Russia declare war against each other, France orders a general mobilization, and the first German army units cross into Luxembourg in preparation for the German invasion of France. During the next three days, Russia, France, Belgium, and Great Britain all lined up against Austria-Hungary and Germany, and the German army invaded Belgium. The "Great War" that ensued was one of unprecedented destruction and loss of life, resulting in the deaths of some 20 million soldiers and civilians.