Three days after Germany announced the resumption of unrestricted submarine warfare on January 31, 1917, the United States broke diplomatic relations. Less than three months later, on April 6, it declared war. Germany, however, still hopeful that the Allies could be beaten before the weight of the United States could be felt, did not declare war in return, at least not immediately. The Germans hoped that when the Allies had been defeated, Germany and the United States could be reconciled without further hostilities.
There is no doubt that the action of German submarines was the immediate cause of America's entering the war. As seen by British eyes,
"the U-boat had proved to be the immediate occasion of one of the greatest events in history — the union of the whole English-speaking world in the struggle for humanity, democracy, and a lasting peace. This is such a wonderful achievement," said the British writer, "that maritime ruthlessness comes almost to wear the appearance of a providential means to an incomparably beneficent end."
As soon as the United States had entered the war, the Allies disclosed to high American officials a secret evaluation of how the war against Germany was actually going. It was a gloomy eye-opening.
"Whenever I think of the naval situation as it existed in April, 1917, I always have before my mind two contrasting pictures," American Admiral William S. Sims wrote, "one that of the British public as represented in their press, and the other that of British statesmen and naval officers."
The public at large was optimistic about the chances of victory. The statesmen and naval officers were pessimistic in the extreme.
"Before arriving in England I myself had not known the gravity of the situation," wrote Sims. "The Germans were not losing the war — they were winning it. The British Admiralty now placed before me facts and figures which disclosed the astounding fact that, unless the appalling destruction of merchant tonnage could be materially checked, the unconditional surrender of the British Empire would inevitably take place within a few months."
Actual shipping losses were three or four times as large as those reported in the press. Britain's First Sea Lord, Admiral John R. Jellicoe, told Sims that "it is impossible for us to go on with the war if losses like these continue."
It is literally true that no American, and very few people in Britain, knew the precariousness of the position of the Allies at the time the United States declared war — precariousness caused in large measure by the submarine.
"It is expressing it mildly to say that I was surprised by this disclosure," said Sims. "I was fairly astounded."
British authorities told Sims that the limit of endurance would be reached on or about November 1, 1917.
"In other words," said Sims, "unless some method of fighting submarines could be found almost immediately, Great Britain would have to lay down her arms before a victorious Germany."
Also fully informed about the gravity of the situation was the American ambassador in London, Walter Hines Page. On April 27, three weeks to the day after the United States entered the war, Page sent a coded message to Washington for the President and the Secretary of State:
There is food enough here to last the civil population only not more than six weeks or two months. Whatever help the United States may render at any time in the future, or in any theater of the war, our help is now more seriously needed in this submarine area for the sake of the Allies than it can ever be needed again, or anywhere else. This seems to me the sharpest crisis of the war, and the most dangerous situation for the Allies that has arisen or could arise.
Sims said some years after the war that
"the world does not yet understand what a dark moment that was in the history of the Allied cause."
Britain, with France, stood tottering on the very brink of defeat, and the principal cause of their disastrous position was the German submarine. But at almost precisely the moment that British leaders could most clearly see a disastrous end ahead, the tide began to turn, slowly and almost imperceptibly at first. The entry of the United States into the war was unquestionably the most important factor in the turn of the tide, but there were other factors, too.
In the field of anti-submarine warfare, the depth charge came into widespread use by April, 1917. German U-boat commanders found this weapon a particularly devastating one both to submarines and to morale. It gave the surface ship a tremendous boost in its anti-submarine efforts.
Probably the most important single factor in the defeat of the German submarine was the convoy system, though. Admiral Sims is often credited with coming up with the idea. He himself later said that he did not do so, but rather gave full support to British naval officers who advocated it.
The advantages of the convoy system were obvious. Instead of large numbers of individual ships sailing singly along the sea lanes, almost all of them far from the protection of friendly warships, the convoy system permitted large groups of otherwise nearly defenseless merchantmen to travel in company with each other under the protection of a number of destroyers and even capital ships.
The disadvantages of the convoy plan were largely imaginary, but they were steadfastly clung to by those who held them — the merchant ship captains themselves. They maintained that no group of merchant ship officers was skilled enough to be capable of sailing in close formation, without lights at night, and — most remote of all — of zigzagging in unison on signal from convoy commanders.
Despite the objections of the captains involved, a single convoy was attempted by a group of ships sailing from Gibraltar to England. The convoy arrived on May 20, 1917, without a collision or a sinking.
"In fine," said Sims, "it meant the Allies could win the war."
Thereafter the convoy system was adopted by almost all merchant ships sailing to or from Britain. In order to sink ships sailing in the protection of a convoy, submarines had to venture within range of prompt attack by destroyers, all of which by now were equipped with depth charges. The submarine loss rate began to climb. In three months from April to June, 1917, twelve U-boats were sunk. In the next three months, twenty were sunk. And in the last three months of 1917 twenty-four were sunk.
Even with its prodigious submarine building program, Germany could not keep pace with this rate of submarine losses. Even more costly was the fact that many of the boats that went down under Allied attack were manned by skilled and experienced crews, headed by seasoned submarine captains. Neither the experience of the crews nor of the captains could be replaced. Green crews and inexperienced captains were less successful in their attacks, and were more likely to fall victim to counter-attacks.
Adding momentum to the turn of the tide against the U-boat was the airplane. Like the submarine, this new weapon had seen very little service prior to the war. By 1917, though, it was — again, like the submarine — a sophisticated weapon of potency and reliability. More than one submarine fell victim to aerial bombs. By the end of the war the Allies had five hundred planes assigned to antisubmarine activities, manned and supported by 2,500 officers and 22,000 men.
In one unusual incident a U-boat was sunk by an aircraft while the submarine was sailing on a joy ride, without orders. The UB-20 sailed on a lark for a short cruise from its base in Flanders in February, 1917, with only half of its crew on board. The purpose of the outing was to impress a group of young German army officers, and to give them something to talk about when they returned to their units. A British airplane caught the U-boat on the surface in water too shallow for a dive to safety, and sank it with a bomb. The body of Lieutenant Commander Glimpf, the U-boat's captain, washed ashore some days later. Rumor had it that if the U-boat had been brought to the surface, the bodies of more than one woman passenger would have been found inside.
Before long the first American troops were on their way to France, through waters patrolled by German submarines. With the advent of the convoy system no American troopship was lost to a submarine, but this did not prevent one soldier from writing to his girl friend a melodramatic account of the dangers on the way, an account that was preserved for posterity by a censor with a sense of humor. The soldier wrote that his ship had fought it out with two or three submarines a day on the way across, that two spies had been found on board and hanged, and
"when we arrived off our port there were no less than eighteen subs waiting for us. Can you beat it?"
The Germans actually were never able to maintain more than fifteen submarines at a time on station in the sea approaches to the British Isles, and usually there were not more than eight or ten. Admiral Sims, the American naval commander, said that if Germany had been able to keep as many as fifty submarines on station "nothing could have prevented her from winning the war."
Fortunately she could not. For every hundred submarines she had in service, only about ten could be on patrol at a time. The rest were either in transit to or from patrols, or in port replenishing supplies, repairing damage, giving their crews needed rest, and accomplishing the routine mechanical maintenance necessary to keep them operating.