Relations between the Blackfoot Confederacy (comprising the Blackfoot, Blood, and Piegan tribes) and whites had been largely hostile for years. Amidst low-level hostilities, sometime in 1869, a rogue Piegan band murdered a white trader. This led to General Philip Sheridan sending out a band of cavalry led by Major Eugene Baker to track down and punish the offending party.
On January 23, 1870, they received a scouting report that the group of Piegans, led by Mountain Chief, was camped along the Marias River. They attacked the site, but Mountain Chief had received warning and left the area, so Baker's men instead ended up attacking the camp of Chief Heavy Runner, who had enjoyed friendly relations with the white men. As the men of the camp were mostly out hunting, the raid was a massacre, killing around 200 women and children. Mountain Chief's band, in the meantime, escaped to Canada.
Responsibility for the massacre and failure to capture Mountain Chief's men was placed by many on Major Baker's well-known alcoholism, though in the subsequent controversy General Sheridan expressed his confidence in Baker's leadership, and managed to prevent an official investigation into the incident.