Final answer:
The decisive victory for the US in the Pacific theater of WWII in 1942 was the Battle of Midway. This battle stopped Japan's advance across the Pacific and shifted the balance of power towards the Allies.
Step-by-step explanation:
The decisive victory for the US in the Pacific theater of WWII in 1942 was the Battle of Midway. This battle took place between June 4th and June 7th and resulted in the sinking of four Japanese aircraft carriers by the US forces. The Battle of Midway was a turning point in the war, as it stopped Japan's advance across the Pacific and shifted the balance of power towards the Allies.