This seriously affected the commerce of the United States, not only with Germany but with neutral countries, such as Holland or Sweden, that could easily transship to Germany the supplies received. Neutral vessels were stopped and taken into Allied ports, there to be detained sometimes for long periods until a decision was reached as to the legality of their traffic. Moreover, the expense of this detention was laid upon the owners of the vessel and cargo. These acts brought forth a series of protests by our government against the policy of the Allies. The correspondence continued with varying results until the United States entered the war. FORCED DECREASE OF NEUTRAL TRADE WITH GERMANY.--Neutral countries adjoining Germany had been making huge profits by selling their food and other products to Germany, replacing their stores with material imported from over seas. As part of the preparation for a long war, the Allies blocked the renewal of neutral stocks of goods. The neutral countries complained vigorously, but they soon cut down their trade with Germany since they were no longer able to replenish their stock of food, rubber, metals, and other supplies. SUBMARINE WARFARE.--In 1914, when the war broke out, Germany is said to have had but four seaworthy submarines. It is difficult to believe that she had so few, but it is certain that she did not have so many as either England, France, or Russia. German naval authorities were not convinced of the value of the submarine in war. However, about a month after the war began, a German submarine torpedoed a British cruiser, and, within a few minutes, two others that had gone to assist the first. Germany, now realizing the value of the new weapon, began the construction of a numerous fleet of underwater boats, or U-boats. But against war ships, properly defended by guns and other means, they proved of little avail after all. Toward the end of the year, Admiral von Tirpitz, head of the German navy, hinted at an extension of the use of submarines to attack merchant ships. Soon numbers of the submarines made their way to the waters surrounding the British Isles, where they torpedoed merchant vessels taking food and supplies to Great Britain and France. The vessels sunk were chiefly British, though some were neutral. PROTECTION AGAINST SUBMARINES.--Large war ships were protected from submarines by keeping them in a mine-protected area until there was need for them at sea. At sea they were protected largely by the patrol and scouting operations carried on by lighter and faster vessels. To reduce the danger to merchant vessels from submarines, harbors and sea lanes were protected by mines and by great nets made of heavy wire cables. The seas in the immediate vicinity of Great Britain were patrolled by thousands of small, swift vessels constantly in search of U-boats.