Battle of the Frontiers

France launched its much-anticipated offensive into Alsace-Lorraine under Plan XVII, while German forces simultaneously invaded Belgium. French infantry, advancing in bright blue coats and red trousers, were mown down by German machine guns and artillery at Morhange, Sarrebourg, and the Ardennes. France suffered 300,000 casualties in three weeks—the bloodiest month in French military history. The offensive's catastrophic failure forced the retreat that ended at the Marne.