What was the turning point for the allies in the asian theater of operations? how did they turn the tide?

The Battle of Midway, June 4-7, 1942.

The Battle of Midway was a "midway" point in the battle against Japan in the Pacific.  It allowed the United States, fighting for the Allies, to go from a defensive position to being on offense.  The battle was one fought mainly with aircraft, and the US was able to knock four of Japan's aircraft carriers out of commission while only losing one of their own aircraft carriers.  Japan's advance across the Pacific halted at Midway, and US forces began its own offensive to retake islands and ultimately pursue the war all the way to Japan itself.