Meanwhile, on 6 April the United States declared war on Germany. In early 1915, following the sinking of the , Germany had stopped its unrestricted submarine warfare in the Atlantic because of concerns of drawing the United States into the conflict. With the growing discontent of the German public due to the food shortages, however, the government resumed unrestricted submarine warfare in February 1917. They had calculated that a successful submarine and warship siege of Britain would force that country out of the war within six months, while American forces would take a year to become a serious factor on the Western Front. The submarine and surface ships had a long period of success before Britain resorted to the convoy system, bringing a large reduction in shipping losses.