I think the question, what constitutes a massacre is a good one. I think one can substitute the word atrocity in its place depending on the circumstances. Massacre means: (the wanton killing of esp. unresisting human beings, inflicting of great damage and defeat), Webster’s. There are elements or arguments for placing Poison Springs, Ft. Pillow, and the “Crater” battle of the Petersburg campaign, into the first definition of what massacre means. Saltville is debatable in that it is better termed an atrocity or just plain murder. The number dealt with in the Saltville incident ranges from 5 to 45 plus. The former battles have greater losses and different circumstances. In the Ft. Pillow and “Crater” incidents, there were also Black Soldiers who fought to the death. They did not separate themselves from those who were surrendering. Those who resisted got shot down along side those who did not. Again here Rebel soldiers took personnel glee in gunning down the Black troops caught in the “Fish Bowl” that was the “Crater”. They determined not to distinguish between those who had surrendered and those who had not. The same took place at Ft. Pillow. The tragedy of Poison Springs is the desecration of the Black infantrymen who lay slain on the field of battle. Port Hudson, Olustee, and Milliken’s Bend were not massacre’s they were defeats. There were recorded individual atrocities or acts committed on the field, but not to the scale of the former mentioned. Milliken’s Bend however was a Federal victory.

Enter the battle of Isandlwana, 1879, where over 1300 British troops and their African allies were killed in one of the worst disasters of the Colonial era, wiped out, by the Zulu army after they arrogantly marched into Zululand. In 1866, Captain William Fetterman rides out of Ft. Phil Kearney vowing to “wipe out the whole Sioux Nation with just 80 men”. He and all of his men fall victim to Chief Red Cloud and his band of Sioux. Fetterman is lead into the trap by a young Oglala warrior named “Curly”. This same warrior would rally the Sioux and its warriors of the “Strong Heart Society” against another candidate of the latter definition of the term massacre. He would then be known to the slaughtered as Crazy Horse. In June of 1876, Lt Col. George Armstrong Custer and one of three battalions of the 7th US Cavalry are killed to a man on the rolling hills of the Little Big Horn Valley. Custer’s loses range from 210 to 250 killed in action. In these three latter instances, the Units in question fought and were defeated to “a man”. A complete and utter victory. A total loss of life for the defeated. “No one left to tell the tale”.
On February 23, 1836, General Antonio López de Santa Anna and his army lay siege to the Alamo and its small band of defenders. Undaunted, the Texans and Tejanos defend the Alamo and hold out for 13 days. The final assault would occur before daybreak on the morning of March 6, 1836. Jim Bowie, renowned knife fighter, and David Crockett, famed frontiersman and former congressman from Tennessee along with some 200 men would die. Defeat would be total and final.
Sand Creek, Washita and Pine Ridge, were massacre’s of the Native population that can be termed atrocities. At Sand Creek in 1864, troops under the command of Col. J.M. Chivington attack and destroy the Cheyenne camp of Chief Black Kettle and Chief White Antelope on Sand Creek, 40 miles from Fort Lyon, Colorado Territory. Black Kettle's band flew an American flag along with a white flag, and considered them selves at peace and under military protection. This did not prevent the terrible slaughter of Women and children, as well as the few warriors available to defend against the attack.
The Washita, November 28, 1868. George Custer and 800 troopers of the 7th Cavalry attack the Cheyenne encampment of Chief Black Kettle and Arapaho Chief Big Mouth. An estimated 51 lodges were destroyed, dozens of men, women, and children brutally slain. This time Chief Black Kettle would not survive the treachery of the US government, his blood would mingle in the Washita along with that of his wife. Custer boasted his body count to be in the hundreds. However historical evidence shows that the actual count was closer to thirty.
On December 29, 1890, the Sioux Chief Big Foot and some 300 of 350 of his followers would be slaughtered by the now infamous 7th cavalry as they camped on the banks of Wounded Knee creek. Chief Big Foot would be numbered among the dead.

Mountain Meadows, Southern Utah, September 8-14, 1857, at least 127 men, women, and children were massacred by order of the leadership of the Mormon Church. They are killed by their own people as well as Natives allied with them. As in previous massacre’s, the slaughtered fought valiantly, but were outnumbered, outgunned and overwhelmed. No quarter was given.

1944, the Ardennes, elements of the 1st SS Panzer Division (who were involved in the German Counter Offensive which became known as known as the Battle of the Bulge), while moving through Belgium would captured and subsequently kill nearly 80 US prisoners of war. They would leave their bodies in the snow to be discovered by allied forces, only after the counter offensive had been beaten back. The slain were captured, disarmed and executed as they huddle together in a cold field.

My lai, Vietnam, as well fits snuggly in the first definition.
Vietnam, March 16, 1968, while on a search and destroy mission, Charlie Company, 11th Brigade, Americal Division enters into the village of My Lai and commences to slaughter whole sale its inhabitants, men, women and children. Without it seems any just cause. It is estimated some 500 civilians lost their lives that day.

And the list is long…

Massacre’s and Atrocities, Massacres or Atrocities or just plain old murder….You decide.