100 years ago on this day the German Imperial Army attacked the French in a last desperate attempt to win the Great War. The French were well prepared and absorbed the first attack. Then three days later they launched a counter offensive that destroyed the German forces.
The German army found itself in a fighting retreat for 100 days, backtracking over the sites of their victories in 1914 until their eventual surrender.
The 1st Battle of the Marne (The miracle of the Marne) denied the Germans a rapid victory and condemned soldiers to four years of trench warfare. The 2nd Battle of the Marne lifted the war out of the trenches at last.
If you want to book end World War 1, and compare the strategies, tactics, forces, morale, weapons etc at the beginning of the war, and at the end of the war then study these two battles. It took four years and the lives of millions of fighting men to learn the lessons that made 2nd Marne possible for the French.