America's military impact on the war was uneven, though in the end the U.S. presence would prove critical in the Allies' push to victory. The first six American destroyers arrived in European waters on May 4, 1917, less than a month after the declaration of war. They went into action immediately— and in close cooperation with Britain's Royal Navy—to help stop the devastating German submarine carrjpaign. But American soldiers arrived in large numbers only after a year's delay. U.S. participation was weakened by problems in command. Reluctant to become too entangled in European aff"airs even as American troops were dispatched overseas, the United States technically did not join the Allied alliance or integrate its armed forces into a unified command. The American army, under the command of General John "Black Jack" Pershing, was engaged as the American Expeditionary Force (AEF) and sought to operate in its own sector of combat. The American forces would not be incorporated into a coordinated Allied military front until late 1918. The importance of the American army's role in the war remains controversial. As the last German offensive took place in June 1918, bringing General Ludendorff's troops dangerously close to Paris, first five, then five more American divisions went into action in the Second Battle of the Mame. That they helped to stop the enemy advance is beyond dispute. Nonetheless, some historians consider that the Germans already had overextended themselves, and that even in the absence of American reinforcements the British and French could have held them. Similarly, the greatest American operation of the war, the Meuse-Argonne offensive from September to the November Armistice, had ambiguous results. General Pershing's hopes that Americans could achieve the breakthrough that had eluded Anglo-French forces for four years proved false. The slow and costly advance by his two field armies served instead to pin down German divisions while the French and British thrust forward in other sectors. Thus, both the American naval effort and the American role in the land war were less decisive than U.S. leaders might have wished. Nevertheless, the psychological effect of a growing American military presence on Germany's armed forces and its home front was inevitably weighty. And, unlike the countries alongside which it fought, the United States found its military strength growing explosively as the war ended. In the final months of the war, for example, more than a quarter million American troops were arriving in France each month—with millions more ready to follow. The AEF had forty-two divisions at the close of the war; in 1919 that number was slated to rise to eighty divisions, surpassing in size both the British and the French forces on the western front.