What was the significance of the battle of the marne?

During World War I, the Battle of the Marne was significant in defining what kind of war these countries were actually fighting. Germany had been following something known as the "Schlieffen Plan", which called for attacking France first to secure a victory that would neutralize the Western Front and free the German army to fight Russia in the East. Supposedly, the France would expect an attack through Alsace-Lorraine, but the Germans would instead invade through Belgium and sweep down through France to fight a battle in Paris.

The Germans were on track until they were stopped by the Britain and France just thirty miles outside of Paris at the Marne River. Britain and France launched a successful counteroffensive and the German line retreated to the Aisne River, destroying the Schlieffen Plan. Unable to advance after the Marne, the armies tried racing one another to the sea. Germany set up a defensive position that the Allies could not break. Along this immovable front (stretching over 400 miles from Switzerland to the English Channel), the Great Powers began what we know today as "trench warfare". This is what many historians call the real start of the war.

The Marne, as a battle alone, upended all of Europe's expectations of the war and demolished hopes that it would finish quickly. The war of movement had stopped. A sort of stalemate had been created after the Battle of the Marne. Thus, the significance of the Battle of the Marne was its defining the World War as not simply a traditional, quick-to-end war, but as one that could potentially turn out long, costly, and deadly.