Japan's involvement in World War II stemmed from its territorial ambitions in Southeast Asia and the South Pacific. The nation was bitter about its failure to gain control of China after World War I. Unwilling to give up their expansionist aspirations, Japan invaded Manchuria in 1931, claiming a minor skirmish with Chinese troops as justification. The Japanese took over China's main ports in 1937 and 1938. Several years later, Vichy France agreed — under coercion — to allow Japanese occupation of northern Indochina. On September 27, 1940, Japan officially threw itself into the growing international conflict by joining the Axis powers.
After signing a neutrality agreement with the U.S.S.R. in 1941, Japan turned its sights to Southeast Asia. Japanese forces took southern Indochina on July 23, prompting the U.S., Britain, and the Netherlands to freeze Japan's assets — a move which would eventually cripple Japan's military by cutting off its access to oil. In response, Japan determined to move further into Southeast Asia, eying the petroleum and other resources available in the region. They also attempted to have the oil embargo lifted — while still maintaining their freedom to have the territory they wanted. General Tojo Hideki, the newly-elected Japanese premier, set a secret November 29 deadline for negotiations to conclude. When this date was missed, Japan moved to carry out an ambitious war strategy. On December 7, 1941, Japanese aircraft struck Pearl Harbor, definitively bringing the United States into the war.
While Japan expanded rapidly throughout the Pacific over the next five months, two significant battles began to turn the tide. American intelligence allowed the Navy to discover and respond to the enemy's moves, giving them an element of surprise. In the Battle of the Coral Sea on May 7-8, they were able to repel Japanese forces at Port Moresby. Less than a month later, in the Battle of Midway, the Navy realized a dramatic victory after sinking four enemy aircraft carriers, part of a powerful naval force steaming towards Midway. Yamamato Isoroko, the commander in chief of Japan's navy, ordered a retreat on June 5; the Japanese would never recover.
On August 7, U.S. Marines landed on the island of Guadalcanal with little initial resistance. Japan quickly retaliated, however, bringing a steady assault by sea and air. Between August and November, six major battles were fought in the waters surrounding the island. Both sides suffered heavy losses. As the larger nation, the United States eventually triumphed in this battle of attrition.
American forces steadily advanced up the recently-occupied island chains toward Japan. In 1944, the Allies successfully invaded the Philippines. The following year saw the Battles of Leyte Gulf, Iwo Jima, and Okinawa. The losses at Okinawa were staggering, with military losses of over 100,000 Japanese and nearly 13,000 Americans, in addition to equally devastating numbers of civilians. In light of these stunning tolls, the dreadful thought of Allied forces doing battle on Japanese soil greatly lessened resistance to the use of the recently-developed A-bomb.
In early August of 1945, American bombers dropped atomic bombs on Hiroshima and Nagasaki. At the same time, Russia declared war on Japan and invaded Manchuria. The staggering casualties inflicted by these final assaults drove Japan to announce its surrender just days later. On September 2, 1945, formal papers were signed which officially ended World War II.
For more detailed information about World War Two operations in the Asia Pacific region, you may wish to read Wikipedia's Pacific War article and study these World War 2 in Asia and the Pacific timelines.