Final answer:
The My Lai Massacre was the event that shocked America and turned many against the military, revealing deep divisions and sparking widespread anti-war sentiment.
Step-by-step explanation:
The event that shocked America and turned many against the military was the My Lai Massacre (C). The My Lai Massacre was a horrific incident during the Vietnam War in which US Army forces killed hundreds of unarmed Vietnamese civilians, mostly women, children, and elderly people in the hamlet of My Lai, on March 16, 1968. The military's initial attempt to cover up the massacre worsened the situation when the truth eventually emerged, leading to public outrage and a trial. Lieutenant William Calley, the lowest-ranking officer, was convicted, however, his sentencing and the circumstances surrounding the massacre polarized American opinion. Some saw Calley as following impossible orders while others saw this as indicative of broader crimes committed by the military hierarchy, highlighting a deeper moral crisis within the war. This event, along with others such as the Tet Offensive, significantly eroded public confidence in the Vietnam War effort and contributed to the growing anti-war sentiment in the United States.