What happened at My Lai?

My Lai was a small village in Viet Nam that Americans believed to be a center of the Viet Cong (communist military) activity. There, on May 16, 1968, US soldiers of the Charlie Company (an infantry unit) engaged in one of the worst war atrocities in US history, killing 504 people—women, children, babies, and the elderly. Women and girls were raped and mutilated before being slaughtered. The massacre was covered up for more than a year.
The massacre, while inexcusable, grew out of plummeting morale among US troops over how the war was being handled and frustrations over the Viet Cong's Tet offensive, which increased US troop casualties, including in the Charlie Company.
The army command ordered the My Lai village to be razed, fearing it was a Viet Cong hotbed. Although it seemed like a peaceful village when US troops arrived, commander William Calley ordered his men to begin shooting and burning out the villagers.
The villagers were unarmed, tried to flee, and offered no resistance to the US troops, who suffered no casualties. Before Hugh Thompson, a US army helicopter pilot, intervened to protect the remaining fleeing villagers, 182 women, 117 children, and 56 infants had been killed.
The massacre was covered up until Seymour Hersh, a journalist, revealed it in November 1969, due to the persistent effort of US solider Ron Ridenhour, who was horrified by reports of the massacre.