If it has to be one of these two battles, it can only be Verdun. Verdun was a battle in which the French army did suffer massive losses. In this battle, at least 360,000 French soldiers were killed and more than another 200,000 were wounded. This was clearly a much worse battle for the French than the Battle of the Marne, in which "only" 80,000 or so French and British soldiers were killed. In addition, the Battle of the Marne was a much more clear-cut victory for the Allies. That battle stopped the initial German attack at the beginning of the war and pushed the Germans back, thus setting the stage for the trench warfare that typified the Western front in this war.