Final answer:
The My Lai Massacre is associated with a shocking incident of U.S. soldiers killing unarmed civilians during the Vietnam War, which became public in 1969 and led to widespread antiwar sentiment and a reevaluation of the ethics of military conduct.
Step-by-step explanation:
The My Lai Massacre was a tragic event during the Vietnam War on March 16, 1968, where U.S. soldiers killed a large number of unarmed My Lai villagers, mistaking them for enemy forces. The event did not reach public knowledge until November 1969, reported by journalist Seymour Hersh. The investigation and trial that followed, specifically the conviction of Lieutenant William Calley, highlighted the polarized views of Americans towards the war and raised questions about military conduct and the morality of the conflict.
The massacre became a symbol of the dehumanizing effects of the war, contributing to the antiwar movement. Details came to light despite initial attempts by the U.S. military to conceal them. Images and testimonies from returning veterans intensified domestic disputes about the nature of U.S. involvement in Vietnam. These reactions solidified when the military conceded the massacre resulted in the deaths of over 300 innocent civilians, further fuelling public outcry and shaping the U.S.'s societal and political discourse about the war.