On October 21, 2016 the U.S. Navy’s Third Fleet returned to the West Pacific. This was the first time the U.S. Third Fleet had sent warships into the Western Pacific since late 1945. That was when the Third Fleet was deactivated. It was reactivated in 1973 to look after the eastern and northern Pacific Ocean areas including the Bering Sea, Alaska, the Aleutian Islands and a sector of the Arctic. During the nearly three years (1943-45) it existed during World War II it was responsible for naval operations in the Central Pacific as far west as the China coast and the South China Sea.
Since late 1945 the Seventh Fleet, based in Japan, has controlled American naval operations in the West Pacific. But now the Third Fleet will, in effect, deal with the western portions of the Central and South Pacific while Seventh Fleet the deals with North Korea, northern China and Russia. What brought the Third Fleet back in October was the need to carry out another FONOPS (freedom of navigation operations). This one was off the Paracels and other islands in the South China Sea. This was necessary to assure freedom of movement through international waters. China protested but did not oppose the American destroyers carrying out the FONOPS. These operations are needed to affirm that many of the Chinese claims to the entire South China Sea are invalid that that the right to free passage through China’s EEZ is assured. By international law (a 1994 treaty), the waters 360 kilometers from land are considered the Exclusive Economic Zone (EEZ), of the nation controlling the nearest land. The EEZ owner can control who fishes there, and extracts natural resources (mostly oil and gas) from the ocean floor. But the EEZ owner cannot prohibit free passage, or the laying of pipelines and communications cables. The U.S. Navy conducts dozens of these FONOPS each year worldwide.
China asserts that it can control who enters its EEZ and harasses American warships and aircraft that do so. China has angered its neighbors by claiming all the islands (especially tiny uninhabited ones) in the South China Sea. This is a 3.5 million square kilometer (1.4 million square mile) area south of China and Taiwan, west of the Philippines and north of Indonesia. China claims the entire area, as if it were one big EEZ. This has aroused the ire of the neighbors, and caused them to unite against China. The U.S. has also said it is not taking sides in the claims over disputed islands and rocks but that freedom of navigation is a primary mission of the American fleet and dozens of such missions are carried out all over the world every year. The U.S. is not giving China any exceptional treatment but is simply pointing out that China is attempting to violate international law.
The expansion of Third Fleet operating areas was no surprise. In 2012 the U.S. announced that it would have 60 percent of its 270 warships in the Pacific by the end of the decade. Actually, this is just a continuation of a process that began when the Cold War ended in 1991. But these changes move slowly. Largely this is the result of political problems that arise when you try to transfer the home ports (where the ships are when not at sea and where the families of the crews live and spend their money) from one coast to another. The politicians representing states on the east coast raise a major stink when the navy tries to move the home ports. It's taken the navy a decade to muster the political clout to make the changes happen. Meanwhile, more and more ships based in east coast ports were serving temporarily in the Pacific or Middle East. Now the big shift has been taking place officially. There have been other indicators that this was happening.
For example in 2006 the U.S. Navy eliminated the Atlantic Fleet, after a century of existence. First established in 1906, the Atlantic Fleet was the first, world class, high seas, naval force from the Americas. At the time there was fear that Germany's ambitious warship building program might someday endanger the United States. The Atlantic Fleet did go to war with the Germans in 1917, and again in 1941.
After 1945, the Atlantic Fleet remained a mighty force, in preparation for a potential battle with the growing naval power of the Soviet Union. But when the Soviet Union dissolved in 1991, their fleet wasted away within a decade. So the American Atlantic Fleet no longer had a major opponent. Meanwhile, China, North Korea, Iraq, Afghanistan, and Iran provided plenty of work for the Pacific Fleet (which normally supplied ships for Middle East and South Asian emergencies).
The Pacific Fleet still had a full plate after 1991, so the Pacific Fleet remained. The Atlantic Fleet was actually renamed, and reorganized, into the U.S. Fleet Forces Command, which will be responsible for the training, maintenance, and operation of naval forces (ships, aircraft, and land installations) on both coasts. The Pacific Fleet will still stand ready to deal with potential problems in Asia.
Actually, the Atlantic Fleet did have its name changed once before, in 1922, to "Commander Scouting Force." It was changed back to Atlantic Fleet in 1941, just in time to fight the Germans once more. But the Russians are not expected to be a threat again, at least not any time soon.
For most of the past century, the Atlantic and Pacific Fleets were basically the two major parts of the U.S. Navy and each developed unique customs. Sailors would often spend their entire careers in one fleet or the other. But when one was transferred, it was immediately apparent, once the transferred sailor arrived at the new location, that the two fleets were quite different. From now on, however, there will be the Pacific Fleet and "the rest of the navy."