The war in the Pacific really starts with the Japanese attack on the US military base in Pearl Harbor on December 7, 1941. After this attack, the US declares war on Japan.
After this event, the the US and Japanese faced off in the Pacific theater (aka the Pacific Ocean). The US had to prepare for this war in the Pacific by moving troops to strategic locations. On the other hand, Japan pressed forward going further east into the Pacific while having some success against the US military.
This success against the US would not last long, as the US gained important victories at the Battle of Coral Sea (May 1942) and Battle of Midway (June 1942).