Sibley's brigade marched west, establishing a base near Devil's Lake in northeastern Dakota Territory, while Sully's men marched north into Dakota Territory along the Missouri River. Sibley made contact with the Santee in the area and arranged for a conference with Santee leaders Standing buffalo and Sweet Corn on July 24 at Big Mound, south of Devil's Lake.
An aura of tension and suspicion hung over the meeting, so when a nervous Santee shot and killed a militiaman he thought was threatening him, violence erupted. Given that there were 2,000 troopers and 1,500 Santee at Big Mound that day, the casualty figures were minuscule. The Santee, who were outgunned by the troops with their artillery, lost forty, but Sibley lost just four, the man shot in the beginning, two in the battle and a fourth who was killed by a lightning strike.
Most of the Santee escaped, and Sibley gave chase. He would not catch them, but on July 26, he crossed paths with another mixed Sioux gathering of about 1,650 Santee and Lakota near Dead Buffalo lake, who promptly attacked the advance guard of Sibley's column with aggressive flanking maneuvers. As had happened at Big Mound, the Sioux broke off the attack after a fierce firefight. Also as at Big Mound, the casualties were tiny compared to the number of combatants on the field. Only one soldier and fifteen Sioux were killed in action.
Two days later, as Sibley's command camped near Stony Lake, they were ambushed by an estimated 2,500 Sioux. It was a determined attack that might have succeeded had not artillery been brought into play. Again, as at the two major battles earlier in the week, casualties were surprisingly light, with none among the Minnesotans, and an estimated eleven among the Sioux.