What two events recast the u. s. attitude toward the war in indochina? why?

Those two events would be :

- The Tet Offensive
where the Forces from north Vietnam launched a surprise attack To American soldiers in South vietnam

- My Lai Massacre
Where the communist Forces gunned down Women , children, and Elderly

As for why, simply because the U.S doesnt want this communist Forces to became the new power in indochina