America Exceptionalism has a glorious history of mass killings. There are so many massacres to choose from that, to limit my choices somewhat, I only looked at racial or religious massacres that had at least the tacit approval of local authorities. Here are some of the most interesting.
Clear Lake Massacre
A regiment of the U.S. Cavalry was sent to find the killers of Kelsey and Stone. When they came upon a Pomo fishing settlement on Clear Lake the Cavalry attacked, indiscriminately killing up to 100 men, women, and children.
Mountain Meadow Massacre
Into this mess came the Fancher Party from Arkansas. A prominent had been murdered around this time and religious elders in Cedar City concluded that people in the Fancher wagon train had done the killing. Some Paiute with Mormon militiamen dressed as Indians attacked the wagon train and held it siege for five days. The Mormons then proposed to give the party safe passage; in exchange had to surrender their weapons, wagons, and cattle.
The men were separated from the women and 17 very young children were put in a wagon. They were marched several miles towards Cedar City. At a prearranged signal the Mormon militiamen turned on the men and killed them all. They then attacked and killed all the women. All totaled, 140 men and women were butchered. The 17 children were taken to Cedar City and passed out as prizes to Mormon families. Brigham Young later told the leader of the massacre, "I asked the Lord if it was all right for the deed to be done...It is all right."
Tulsa Race Riot