The Pacific Theater is the area in and next to the Pacific Ocean, in which World War II was fought. There are two sub-regions of the Pacific Theater which are the South West Pacific Theater and the Pacific Ocean Theater. The main combatants of the Pacific War were the United States, Japan, Australia, Great Britain, and China.
It is generally considered that the Pacific War began on the 7th of December 1941 with the Japanese invasion of Thailand (then called Siam), the invasion of British Malaya and the Bombing of Pearl Harbor and later the declaration of war on the 8th. (Japan also bombing U.S Navel bases at Guam and Wake Island).
With British, Australian and Dutch forces, already occupied fighting Germany, and heavily committed in the Middle East, North Africa and elsewhere, were unable to provide much more than token resistance to the battle-hardened Japanese. The Allies suffered many defeats in the first six months of the war. Two major British warships, HMS Repulse and HMS Prince of Wales were destroyed by a Japanese air attack off Malaya on 10 December 1941.
Thailand surrendered within twenty-four hours of the Japanese invasion. Thailand formally allied itself with Japan on 21 December.
Hong Kong was attacked on 8 December and fell on 25 December 1941, with Canadian forces and the Royal Hong Kong Volunteers playing an important part in the defense. U.S. bases on Guam and Wake Island were lost at around the same time.