The Royal Naval Air Service (RNAS) was the air arm of the Royal Navy, under the direction of the Admiralty's Air Department, and existed formally from 1 July 1914 to 1 April 1918, when it was merged with the British Army's Royal Flying Corps to form the Royal Air Force (RAF), the world's first independent air force. It was replaced by the Fleet Air Arm, initially consisting of those RAF units that normally operated from ships, but emerging as a separate unit similar to the original RNAS by the time of World War 2.