On board an American destroyer. A five inch gun and its crew. United States Navy operations during World War I began on April 6, 1917, after the formal declaration of war on Germany. The American navy focused on countering enemy U-boats in the Atlantic Ocean and the Mediterranean Sea while convoying men and supplies to front lines in France and Italy. Because of United States' late entry into the war her capital ships never engaged the Germans and few decisive submarine actions occurred. Five inch 51 caliber guns formed the main battery of the first United States Navy light cruisers and the secondary batteries of United States Navy battleships built from 1907 through the 1920s. United States naval gun terminology indicates the gun fired a projectile 5 inches (127 mm) in diameter, and the barrel was 51 calibers long. Photo taken by Central News Photo Service, circa 1918.