Convoys worked for a number of reasons. Concentrating vessels in convoys made them less visible to a submarine than they would have been as individual travelers, and submarine commanders complained that the ocean now seemed empty. There was little visibility from a submarine's periscope, and, instead of an endless stream of individual ships to watch for, the captain of the submarine now had the far less frequent opportunity of spotting a convoy. When German attackers encountered convoys and their armed escorts, only a brief assault was possible before the submarine itself was placed in deadly peril as several enemy destroyers pummeled it with depth charges. Contrary to the view of conservative British admirals, the convoy forced the submarines to fight the British navy—and on highly unfavorable terms. The best German commanders, or at least the most aggressive, suffered the highest losses. The Germans discovered that the entry of the United States into the war—a certainty once Berlin ordered unrestricted submarine warfare—injured their efforts quickly and painfully. The rapid arrival of more than thirty American destroyers in Europe was a key factor in supplying an adequate number of escorts. The United States seized interned German cargo vessels when war was declared: thus, half a million tons of shipping from the merchant German fleet shored up the beleaguered Allied forces. By September 1917 the tide had turned. Most shipping in the busy North Atlantic corridor went by regular convoy. Each month, more of the Allied sea traffic came under the convoy system. Losses to the submarine continued, but at a sharply reduced rate. By the war's last months, German submarines were sinking only one merchant vessel for every four they had destroyed in spring 1917. Frustrated commanders returned to port with most of their torpedoes still on board.