In 1911, Churchill was transferred to the office of the First Lord of the Admiralty, a post he held into World War I. This was the year of the Agadir Crisis, with which Churchill opens "The World Crisis", his account of World War One. His first major act was to replace all but one of the Sea Lords, the senior naval officers who administered the Admiralty. With the aid of the new First Sea Lord Sir Francis Bridgeman he created a war staff, gave impetus to reform efforts, including development of naval aviation (he undertook flying lessons himself), the use of the 15" gun as the main armament of battleships, the development of the fast battleship (which found shape in the "Queen Elizabeth" class) and of the 6" gunned light cruiser (which found shape in the "Arethusa" class) concepts, and the switch from coal to oil in the Royal Navy, a massive engineering task, which depended on securing oil supplies in Mesopotamia. In 1912, in response to the German Naval Law of that year, Churchill brought forward Naval Estimates based on a principle of building two British battleships to every one German, which became known as 'two keels to one."