Battle of the Marne (1914)

The First Battle of the Marne (September 1914) stopped the German advance through France that had been so rapid it threatened Paris. Joffre's counter-attack — famously using 600 Paris taxicabs to rush reinforcements to the front — forced German forces to retreat to the Aisne river, ending the Schlieffen Plan and condemning both sides to four years of trench warfare. Had Germany broken through, World War I might have ended in months.