Ten days after the attack on Pearl Harbor on December 7, 1941, Nimitz was selected by President Franklin D. Roosevelt to be commander-in-chief, United States Pacific Fleet (CINCPACFLT).
He was promoted to the rank of admiral, effective December 31, 1941.
Nimitz immediately departed Washington for Hawaii and took command in a ceremony on the top deck of the submarine Grayling.
The change of command ceremony would normally have taken place aboard a battleship, but every battleship in Pearl Harbor had been either sunk or damaged during the attack.
Assuming command at the most critical period of the war in the Pacific, Admiral Nimitz organized his forces to halt the Japanese advance, despite the shortage of ships, planes, and supplies.
 He had a significant advantage in that the United States had cracked the Japanese diplomatic naval code and had made progress on the naval code JN-25.
The Japanese had kept radio silence before the attack on Pearl Harbor, but now events were moving so rapidly that they had to rely on coded radio messages that they did not realize were being read in Hawaii.

On March 24, 1942, the newly formed US-British Combined Chiefs of Staff issued a directive designating the Pacific theater an area of American strategic responsibility.
Six days later, the US Joint Chiefs of Staff (JCS) divided the theater into three areas: the Pacific Ocean Areas, the Southwest Pacific Area (commanded by General Douglas MacArthur), and the Southeast Pacific Area.
The JCS designated Nimitz as "Commander in Chief, Pacific Ocean Areas", with operational control over all Allied units (air, land, and sea) in that area.

Nimitz, in Hawaii, and his superior Admiral Ernest King, the Chief of Naval Operations, in Washington, rejected the plan of General Douglas MacArthur to advance on Japan through New Guinea and the Philippines and Formosa.
Instead they proposed an island-hopping plan that would allow them to bypass most of the Japanese strength in the Central Pacific until they reached Okinawa.
President Roosevelt compromised, giving both MacArthur and Nimitz their own theaters.
The two Pacific theaters were favored, to the dismay of generals George Marshall and Dwight Eisenhower, who favored a Germany-first strategy.
King and Nimitz provided MacArthur with some naval forces, but kept most of the carriers.
However, when the time came to plan an invasion of Japan, MacArthur was given overall command.

Nimitz faced superior Japanese forces at the crucial defensive actions of the Battle of the Coral Sea and the Battle of Midway.
The Battle of the Coral Sea, while a loss in terms of total damage suffered, resulted in the strategic success of turning back a Japanese invasion of Port Moresby on the island of New Guinea.
Two Japanese carriers were temporarily taken out of action in the battle, which would deprive the Japanese of their use in the Midway operation that shortly followed.
The Navy's intelligence team figured that the Japanese would be attacking Midway, so Nimitz moved all his available forces to the defense.
The severe losses in Japanese carriers at Midway affected the balance of naval air power during the remainder of 1942, and was crucial in neutralizing Japanese offensive threats in the South Pacific.
Naval engagements during the Battle of Guadalcanal left both forces severely depleted.
However, with the allied advantage in land-based air power, the results were sufficient to secure Guadalcanal.
The US and allied forces then undertook to neutralize remaining Japanese offensive threats with the Solomon Islands campaign and the New Guinea Campaign, while building capabilities for major fleet actions.
In 1943, Midway became a forward submarine base, greatly enhancing US capabilities against Japanese shipping.

In terms of combat, 1943 was a relatively quiet year, but it proved decisive inasmuch as Nimitz gained the materiel and manpower needed to launch major fleet offensives to destroy Japanese power in the central Pacific region.
This drive opened with the Gilbert and Marshall Islands campaign from November 1943 to February
Ten days after the attack on Pearl Harbor on December 7, 1941, Nimitz was selected by President Franklin D. Roosevelt to be commander-in-chief, United States Pacific Fleet (CINCPACFLT). He was promoted to the rank of admiral, effective December 31, 1941. Nimitz immediately departed Washington for Hawaii and took command in a ceremony on the top deck of the submarine Grayling. The change of command ceremony would normally have taken place aboard a battleship, but every battleship in Pearl Harbor had been either sunk or damaged during the attack. Assuming command at the most critical period of the war in the Pacific, Admiral Nimitz organized his forces to halt the Japanese advance, despite the shortage of ships, planes, and supplies.  He had a significant advantage in that the United States had cracked the Japanese diplomatic naval code and had made progress on the naval code JN-25. The Japanese had kept radio silence before the attack on Pearl Harbor, but now events were moving so rapidly that they had to rely on coded radio messages that they did not realize were being read in Hawaii.
On March 24, 1942, the newly formed US-British Combined Chiefs of Staff issued a directive designating the Pacific theater an area of American strategic responsibility. Six days later, the US Joint Chiefs of Staff (JCS) divided the theater into three areas: the Pacific Ocean Areas, the Southwest Pacific Area (commanded by General Douglas MacArthur), and the Southeast Pacific Area. The JCS designated Nimitz as "Commander in Chief, Pacific Ocean Areas", with operational control over all Allied units (air, land, and sea) in that area.
Nimitz, in Hawaii, and his superior Admiral Ernest King, the Chief of Naval Operations, in Washington, rejected the plan of General Douglas MacArthur to advance on Japan through New Guinea and the Philippines and Formosa. Instead they proposed an island-hopping plan that would allow them to bypass most of the Japanese strength in the Central Pacific until they reached Okinawa. President Roosevelt compromised, giving both MacArthur and Nimitz their own theaters. The two Pacific theaters were favored, to the dismay of generals George Marshall and Dwight Eisenhower, who favored a Germany-first strategy. King and Nimitz provided MacArthur with some naval forces, but kept most of the carriers. However, when the time came to plan an invasion of Japan, MacArthur was given overall command.
Nimitz faced superior Japanese forces at the crucial defensive actions of the Battle of the Coral Sea and the Battle of Midway. The Battle of the Coral Sea, while a loss in terms of total damage suffered, resulted in the strategic success of turning back a Japanese invasion of Port Moresby on the island of New Guinea. Two Japanese carriers were temporarily taken out of action in the battle, which would deprive the Japanese of their use in the Midway operation that shortly followed. The Navy's intelligence team figured that the Japanese would be attacking Midway, so Nimitz moved all his available forces to the defense. The severe losses in Japanese carriers at Midway affected the balance of naval air power during the remainder of 1942, and was crucial in neutralizing Japanese offensive threats in the South Pacific. Naval engagements during the Battle of Guadalcanal left both forces severely depleted. However, with the allied advantage in land-based air power, the results were sufficient to secure Guadalcanal. The US and allied forces then undertook to neutralize remaining Japanese offensive threats with the Solomon Islands campaign and the New Guinea Campaign, while building capabilities for major fleet actions. In 1943, Midway became a forward submarine base, greatly enhancing US capabilities against Japanese shipping.
In terms of combat, 1943 was a relatively quiet year, but it proved decisive inasmuch as Nimitz gained the materiel and manpower needed to launch major fleet offensives to destroy Japanese power in the central Pacific region. This drive opened with the Gilbert and Marshall Islands campaign from November 1943 to February 
Chester W. Nimitz