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
In the 1840's American settlers Andrew Kelsey and Charles Stone had taken to kidnapping and enslaving members of the peaceful Pomo tribe. They would freely beat, torture, starve and rape tribe members they forced to work in the gold fields of central California. They would whip Pomo parents who refused to bring their children for Kelsey and Stone to rape. In the Spring of 1850, a couple of Pomo natives freed themselves by killing Kelsey and Stone.
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
In the late 1850's there were many wagon trains traveling through the Utah Territory to California. Mormons under their religious dictator Brigham Young were undergoing a spiritual cleansing that included the murder, or "blood atonement," of sinners within the congregation. At the same time, Mormons were driving any Federal agents from the territory with threats of killing them if they remained. President Buchanan responded by threatening to send in the army to quell the rebellion. Also at this time, Mormon leaders told the Native American Paiutes that Brigham Young was giving the cattle of any wagon trains headed to California to the Paiute.
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."
Lawrence Massacre
In 1863, 400 Confederate militia under the command of the notorious William Quantrill attacked the Jayhawker town of Lawrence, Kansas. The town was safely devoid of Union troops allowing Quantrill's raiders free reign to burn and pillage they town. They rounded up nearly 200 men and boys and shot them down in cold blood.
Louisiana Massacres
In the decade following the Civil War, Louisiana whites waged a war of killings to prevent freed blacks from voting. New Orleans (1866) saw white mobs killed 38. Coushatta (1874) saw the White League attack and kill white Republican officeholders and freed blacks, killing over 20. Colfax (1873) had the local sheriff organize 300 armed whites to attack the courthouse defended by black citizens. After the fighting when the defenders surrendered their arms, the whites engaged in a mass killing of the black defenders, killing at least 100 men.
Tulsa Race Riot
In 1921, the Ku Klux Klan was at its zenith. That year a mob of whites attacked the wealthy segregated black Greenwood District, also known as the Black Wall Street. The mob burned over 1,200 homes and killed as many as 300 people with impunity. When the police finally moved in they arrested over 6,000 black victims, blaming the victims for the violence.