King's savior and resurgence of his career was in part the coming together of 3 different factors meeting the world events of late 1940-1942 and the need for high energy in naval high command.

1. Was his eventually close relationship with FDR, and Secretary Knox.

2. His greatness as a naval officer eventually overrode his personal attitude and demons, and his professional reputation was never ruined.

3. His ability to be the fighter and wartime admiral along with his seemingly limitless reserves of energy.

It is also important to understand the billet of COMINCH, or its previous version, Commander-in-Chief US Fleet, the terribly named CINCUS(King renamed it after Pearl Harbor).

In the interwar period the USN went through several structural reforms, thanks to the Panama Canal it was no longer seen as crucial to maintain equally powerful Atlantic and Pacific Fleets. Thus the force was combined, you had the "Battle Force" made up of the gun line and scouting assets and escorts, and the "Patrol Force" of older and secondary ships to support the main force. The commander of the Battle Force typically dual hatted as CINCUS.

In 1940 Admiral Richardson got into a bad spat with FDR over the decision to move the Battle Force from California to a new main base at Pearl Harbor, he felt it was too exposed and under developed. This led to his term being cut short and a shuffle in high command. It also led to another reform in the fleet structure. The Pacific Fleet and Atlantic Fleets were reformed, while the Asiatic Fleet under Hart had always been a separate side show command. Admiral Kimmel was named CINCPAC, and as the main sea going billet as CINCUS in case the combined force ever did operate together. While King who had tried to get named as CNO had been stuck on the General Board, normally a holding ground till officers had to retire, however Stark who had been named CNO, got him the CINCLANT job.

In that post he was faced with the challenge of a new naval war in the Atlantic, and cemented his place in FDR's mind as one of "his admirals" and a great fighter. This was high praise from the President who loved the Navy most of all, more even than those who had worn the uniform, Asst SECNAV was his longest job besides President itself.

With Kimmel disgraced by Pearl Harbor some additional changes had to be made. Nimitz was sent out to Pearl and told to "not come back until he has won". But the need to organize a global war and coordinate the entire fleet could not happen from a ship. So King is brought in as COMINCH to handle the actual operations of the ships and commands. To "Fight the Navy" as it were in the days before Combatant Commanders and the modern force structure. Stark meanwhile continues as CNO, doing much of what the modern CNO does, long term planning, force management, acquisitions and such.

But Stark's greatest failure was perhaps not his own doing, while yes Pearl Harbor occurred on his watch it was not particularly his undoing. What it was, was that he was simply not Ernie King. King had the work capacity, and force of nature essentially to force a consolidation of the offices, he could do the whole job better than with two splitting the load. He also was able to develop a working relationship with Admiral Leahy in the end, who was FDR's CoS and who had been the CNO before Stark. It also helped that it balanced the Army and Navy 2-2 for the US high command with Marshall and Hap Arnold on the other side.

So he gets both posts, and Stark gets to save face by going out to England.

Ian W. Toll's Pacific Crucible gives a great rundown on the Navy in early WW2 and Ernest King and the other senior Admirals as well.

And Borneman's The Admirals are both great places to start for more.