The Facts Pre-massacre

The My Lai Massacre is a result of the guerrilla warfare that the United States faced during the Vietnam War. 

It took place on March 16th, 1968 in Son My village, Sơn Tịnh District of South Vietnam. The two hamlets of Son My were where the massacred transpired: My Khe and My Lai. 

http://www.cnn.com/WORLD/9803/16/my.lai/

-Who were the perpetrators?
Charlie Company of the 1st Battalion, 20th Infantry Regiment, 11th Brigade, 23rd Infantry Division 

-Why did they go to My Lai?
The Tet Offensive of January 1968 was carried out in Quang Ngai by the 48th Local battalion of the National Liberation Front or the Viet Cong. Intelligence reports from the U.S. suggested that the Viet Cong were taking refuge in Son My. Hamlets My Lai(1) through My Lai(6) were thought to be harboring the 48th (1).

Wanting to regain the initiative, the U.S. sent Task Force Barker, consisting of three rifle companies from the 11th Brigade, including Company C from the 20th Infantry, led by Lt. Col. frank A. Barker (2).

-The Orders?
Task Force Barker was ordered on March 16th-18th to destroy the rest of the 48th Viet Cong. Before the engagement, Col. Oran K. Henderson, the commander of the 11th Brigade, told his troops to "go in there aggressively, close with the enemy and wipe them out for good" (3). Lt. Col. Barker ordered the 1st Battalion Commanders to burn the houses, kill the livestock, destroy food supplies and destroy the wells (4).

-The Discrepancy?
Captain Ernest Medina of the Charlie Company reported to his troops that nearly all of the civilians had evacuated and that those that remained were supporters of the Viet Cong (5). The question was raised about who to kill and does it include women or children. Different responses have been given: 1.kill all North Vietnamese combatants and suspects (including women and children), burn the village, and pollute the well (6).
2."They're all VC, now go and get them", reply to the question "Who is my enemy?" by saying, "Anybody that was running from us, hiding from us, or appeared to be the enemy. If a man was running, shoot him, sometimes even if a woman with a rifle was running, shoot her." (7)
3.At Calley's trial, a defense witness testified that Medina instructing to destroy everything in the village that was "walking, crawling or growing". (8)

-Total Deaths: 
U.S. estimate around 347 deaths
Local Vietnamese government estimates 504 deaths