The Second Battle of the Atlantic was the longest continuous military campaign of World War II, running from 1939 right through to the defeat of Nazi Germany in 1945, and was at its height from mid-1940 through to about the end of 1943. The naval battle pitted the German Navy against convoys from North America to the United Kingdom, protected mainly by the British and Canadian navies and air forces, which were later aided by United States forces. The Germans were assisted by a small number of Italian submarines after the country joined the Axis Powers. Although many ships were sunk, the Allies gradually gained the upper hand.