first answer:
The 1941 Japanese attack on the U.S fleet in Pearl Harbor virtually destroyed it.
second correct answer:
The attack caused great damage, however the US Pacific Fleet overall was very much operational after the attack.
Some Background: In May 1940, President Roosevelt (on the advice of Stanley Hornbeck) ordered the Fleet in the Pacific to move from its base in San Diego to Hawaii, later decided to be at Pearl Harbor by June 1940. In May 1941 about 25% of the Pacific Fleet was transferred to the Atlantic Ocean to assist US efforts to support Britain.
On December 7, 1941 (US & Hawaii date/time), these were the general dispositions of the US Navy:
In the Atlantic: 4 Aircraft Carriers, 1 Escort Carrier, 3 old battleships, 3 battleships, 2 fast battleships, 5 heavy cruisers, 8 light cruisers, 31 old destroyers, 49 destroyers, 50+ submarines.
At Pearl Harbor: 8 battleships, 2 heavy cruisers, 6 light cruisers, 4 old destroyers , 25 destroyers, 4 submarines, & 51 other ships.
At Sea in Pacific: 2 aircraft carriers, 10 heavy cruisers, 2 light cruisers, 4 old destroyers, 12 destroyers, 10 submarines & 28 other ships.
At US West Coast: 1 aircraft carrier, 1 battleship, 1 light cruiser, 9 destroyers, 9 submarines, & 5 other ships.
At Philippines, Borneo & en-route to Singapore: 1 heavy cruiser, 2 light cruisers, 13 old destroyers, 29 submarines & 17 other ships.
The ships that were at sea, were primarily involved in operations to reinforce Midway & Wake Islands, escort convoys to/from the Philippines, or were on patrol in waters not far from Hawaii.
As you can see from the above information, that a large part of the US Pacific Fleet was not at Pearl Harbor on the morning that it was attacked. The major exception, and the primary target of the attack, were the 8 battleships. It was these battleships that suffered most of the destruction and damage inflicted on the ships during the attack. At the time, these losses appeared to be quite severe and the American naval leaders, politicians & public were shocked. However in hindsight, these losses were not as devastating to the American war effort as originally thought.
Here is a brief summary of US ship losses & damage to the 8 battleships at Pearl Harbor:
Battleship-USS Arizona, sunk; 1,103 killed
Battleship-USS Oklahoma, sunk; 415 killed
Battleship-USS West Virginia, sunk; 106 killed
Battleship-USS California, sunk; 98 killed
Battleship-USS Nevada, beached; 60 killed
Battleship-USS Pennsylvania, slight damage; 29 killed
Battleship-USS Tennessee, minor damage; 5 killed
Battleship-USS Maryland, some damage; 4 killed
Only the USS Arizona & USS Oklahoma were total losses. The USS West Virginia, USS California, and USS Nevada were all raised/refloated and then repaired & improved on the US west coast in time to fight in the Pacific war.
The USS Pennsylvania, USS Tennessee, and USS Maryland were all repaired on the US west coast and returned to service in less than four months.
There were several reasons that the loss and damage to the battleships was not as devastating to the US fleet as it otherwise could have been.
Besides the ships not at Pearl Harbor, or not damaged at Pearl Harbor, the US Navy was already expanding. Specifically…
Already under construction at shipyards in the US at the time of the Pearl Harbor attack were:
6 fleet aircraft carriers, 4 light aircraft carriers, 1 escort carrier, 8 fast battleships, 4 heavy cruisers, 11 light cruisers, 6 AA light cruisers, 124 destroyers and numerous submarines.
Additionally the following ships were already funded & ordered to be built soon:
2 light aircraft carriers, 4 escort carriers, 2 fast battleships, 6 fast battle cruisers, 4 heavy cruisers, 12 light cruisers, 2 AA light cruisers, 60 destroyers and more submarines.
The US Fleet was not told of such an attack.
Japan attacked Pearl Harbor to destroy the US Naval Fleet in the pacific. One of the main targets were the Aircraft Carrierswhich were not in Pearl Harbor at the time of the attack.
East coast of US, and at sea. Delivering aircraft to Wake Island
The Japanese Imperial Fleet attacked the US Pacific Fleet at Pearl Harbor.
The American Pacific Naval Fleet.
the pacific fleet was moved
The US Fleet was not told of such an attack.
The attack on Pearl Harbor aimed to cripple the US Pacific fleet. The Japanese were set to sweep into Burma, Malaya, the East Indies, and the Philippines, and were afraid the US fleet would spoil their plans.
Japan attacked Pearl Harbor to destroy the US Naval Fleet in the pacific. One of the main targets were the Aircraft Carrierswhich were not in Pearl Harbor at the time of the attack.
East coast of US, and at sea. Delivering aircraft to Wake Island
They disabled the fleet in the Pacific and could attack the small islands without the worry about the navy.
American aircraft carriers were out at sea at the time of the attack
Pearl Harbor was the main base for the US Pacific Fleet.
The Japanese Imperial Fleet attacked the US Pacific Fleet at Pearl Harbor.
The American Pacific Naval Fleet.
Japanese
Almost all of the US Pacific Fleet was moored in Pearl Harbour.