The United States involvement in the war generally came as a result of tensions in the Pacific and Far East with the Japanese Empire. Japan's advances into China had concerned the West, in addition with Japan's withdrawal from the League of Nations during the 1930s. As such, the United States and others began to freeze exports to Japan in the early 1940s.

This US trade embargo had some impact. Oil especially was something that the Japanese imported as there was precious little oil in Japan. If the embargo continued the Japanese could potentially run their oil supplies dry. Any further negotiation with the United States would probably require Japanese withdrawal from most of China. As such, the Japanese felt that war with the United States was a risk worth taking to expand their empire in the Pacific, gain many additional raw materials, and in addition the oil supplies of the Philippines were an obvious target.

With the Europeans pre-occupied in Europe, the US naval presence was the main obstacle to future Japanese advances in the Pacific. More specifically, the US Fleet stationed at Pearl Harbor included up to eight battleships and four aircraft carriers after Roosevelt had re-enforced the harbor with many additional warships. A singe large air-strike on this fleet could potentially wipe most of it out.

As such, Japan drafted plans for Pearl Harbor.[1] With their carrier fleet the Japanese would approach Pearl Harbor and then target the battleships, aircraft carriers, planes and oil depots at Pearl Harbor. In December of 1941 the Japanese had reached their targets and delivered their payloads. All the battleships at Pearl Harbor were hit to some extent, and Japan did enough to effectively sink five of them. Hundreds of US aircraft were also wiped out. Fortunately for the Americans their aircraft carriers were not in port during the raid. In addition to this, plans for a third Japanese sortie were abandoned which may well have spared the oil depots at Pearl Harbor.

Pearl HarborCredit: This is a public domain image from Wiki Commons.  
The above image is a public domain image from Wiki Commons.  

After this, Japan and the USA declared war on the other. Now the war in the Pacific had begun and the United States was left to ponder how Japan had bombed Pearl Harbor without being detected. The USA was soon joined by European allies such as Britain that had a notable military presence in Singapore.

Months after Pearl Harbor Japan had gained the advantage. Japanese advances in the region overran European colonies in the Pacific, including the British strong-hold of Singapore which resulted in the surrender of over 100,000 British and Commonwealth troops as Malaya fell to the Japanese. In the Philippines, the Americans and General Douglas MacArthur were also pushed out allowing for Japan to seize the oil supplies there. The Japanese advanced close to Australia, leaving the Australian mainland within range of their bombers.

Japan's advance was only checked at sea by the remains of the US Pacific Fleet and their aircraft carriers. At the Battle of Coral Sea the Japanese could not occupy their intended target of Port Moresby and withdrew. Although it was a close battle, the US aircraft carriers had held firm.

Worse was to come at the Battle of Midway later in 1942. Admiral Yamamoto targeted Midway Island as a trap that would lure the US aircraft carriers out to four Japanese aircraft carriers.[2] With this, Yamamoto aimed to sink them and then occupy Midway Island. However, the US intelligence had deciphered the Japanese codes and so Admiral Nimitz was well aware of Japanese plans around Midway Island. As such, he sent his own aircraft carriers to defend Midway Island.

In the battle that followed Japan's aircraft such as the A6M Zero were more than a match for the US planes. However, in one bold sortie a few US SBD dive-bombers broke through and hit three of Japan's four carriers which was enough to sink them. As the battle continued the United States lost one of their own carriers, but the sinking of the fourth and remaining Japanese carrier was suitable compensation. As such, the battle was a big defeat for Japan as they had lost not only four aircraft carriers, but hundreds of supported aircraft, and thousands of personnel.

Undoubtedly, the Battle of Midway was the USA's greatest victory in the Pacific War at least. It had gained the Allies an advantage that they would not let slip. In 1943, Guadalcanal would also fall to the Allies providing an important base for further advances in the region.

From here on, the Japanese were largely in retreat.Douglas MacArthur proposed Operation Cartwheel that become the cornerstone of Allied strategy for the remainder of the war. Overall, the strategy proved effective as the United States defeated the Japanese at Tarawa, in the Marianas, and then re-gained the Philippines in 1944.

During such advances, the US Navy inflicted two further devastating naval defeats on the Imperial Japanese Navy in 1944. Firstly, the Battle of the Philippine Sea resulted in the loss of a further three Japanese aircraft carriers as their aircraft struggled against new Allied planes. Then, later in the year the Battle of Leyte Gulf wiped out much of the remainder of the IJN and left the Allies with clear naval superiority.

As such, by 1945 it was clear enough that the Pacific War was all but won. Japan could only hope to cut its losses, but continued to defend the home islands such as Iwo Jima and Okinawa. Such islands could not be held as the US Army and Marines made steady progress towards Japan.

The USA called for Japan's unconditional surrender. However, at first the Japanese ignored this. With the development of the A-bomb the Americans chose this as an alternative to landing on the Japanese mainland. After two were dropped on Japan, the Japanese surrendered to the United States and the Allies on the USS Missouri in 1945.

Overall, the US involvement in the Pacific War was substantial. Few will doubt that the war in the Pacific could not have been won without them. As such, the Pacific War was primarily America's front while the Chinese and British did what they could to assist.