What two american naval and air victories in mid-1942 stemmed the japanese tide? what island victory early in 1943 ended japanese chances at an offensive toward the south?

The two victories which helped the American side during the World War II were the Battle of Coral Sea where the US forces were able to turn back the Japanese fleet. Another victory was Midway (this is also a very known battle and often considered the tipping point). 

The island victory you're asking about was Guadancanal.