The Pacific War is the theater of WWII that took place in Asia and the Pacific, hence the name.
Three of the main combatants on the Allied side were the United States, Great Britain, Australia, New Zealand and China, while the two Axis powers were the Empire of Japan and the Kingdom of Thailand.
Casualties for the Allies were often high, as the Japanese (The main axis threat in the theater) had acquired skills in fighting in jungle warfare, while the US Marines had not. Also, armor and motorized units could seldom be used because of deep mud, winding or narrow roads (or the lack of roads altogether) and lack of proper supplies and small bridges, which would severely limit their movement. This was also were the majority of naval battles in WWII took place because of the size of Japan's and America's navy, and because control of the seas was critical for keeping a steady flow of supplies and troops to the area. The war resulted in a Japanese defeat as the Americans bombed Hiroshima and Nagasaki with devastating nuclear bombs which eventually forced the Japanese to surrender.
Medal of Honor: Rising Sun was the first Medal of Honor game to take place in the Pacific Theater, and was followed by Medal of Honor: Pacific Assault.