The Marne was the result of Germany's Schlieffen Plan, which attempted to make Germany move into France and across Europe counting on one thing: speed.
This single battle destroyed Europe's hopes of a quick war. The war of movement stopped and was replaced by a new war tactic known as trench warfare, which would make the war much harder to end than anyone could have suspected.
Thus, the battle of the Marne was the real beginning of World War I.