As of August 18, 1862, the fort was garrisoned by 76 men and two officers of Company B of the 5th Minnesota Infantry Regiment, under the command of Captain John S. Marsh, who had fought in the Civil War in the First Battle of Bull Run.

The primary Indian Agent was against it, but the other men persuaded him to give the Dakota a small amount of porkback and flour.

The greatly out-numbered 67 white men gathered there became uncomfortable with the stipulation and began to form small groups to head back to their homes.

[citation needed] Fighting broke out as some of the Dakota men pursued the departing whites, while the rest surrounded those holed up in Agency buildings.

Some white settlers escaped, heading for Fort Ridgely, while the majority tried to race for their homes and families.

[citation needed] Mr. J.C. Dickinson, who seems to have been the first to escape, took his family in a wagon to Fort Ridgely, where nobody believed that there had been an attack.

Marsh ordered Drummer Charles M. Culver, a twelve-year-old (who would die in 1943, at 93, as Company B's last survivor) to beat the long-roll.

[11] The defeat of Marsh and B Company, combined with Sheehan's departure, had left Fort Ridgely severely undermanned, and it had no fortifications.

Oscar Wall ascribes this miscalculation to dissension among the Dakota and their mistaken belief that the fort held more than 100 trained soldiers.

Upon Bishop's report of the ambush at Redwood Ferry after arrival at the fort that night, Bishop and Sheehan sent a Private William J. Sturgis to ride through the night and spread the warning of the uprising, including to Minnesota Governor Alexander Ramsey about the uprising.

[12] Lt. Norman K. Culver, Company B, and others responded to Sturgis's plea by recruiting volunteers in St. Paul, who arrived at Fort Ridgely with the "Renville Rangers" as reinforcements on the evening August 19.

[15] At 1 o'clock in the afternoon of August 20, Little Crow rode out alone into the open beyond the picket line west of the fort, close enough to be recognized but just out of musket range.

After Sergeant Bishop offered to meet him at the picket line rather than rushing out in an attempt to capture him, the diversion was revealed as shots rang out from the wooded ravine near the northeast corner of the fort.

Sergeant Jones, supported by Lieutenants Culver and Gorman and the Renville Rangers, held the Dakota at bay with his six-pound field gun.

Ordnance Sergeant Jones had three six-pound cannons, two twelve pounders, and one twenty-four pound gun positioned.

The first attack came from the southwest and succeeded in capturing two outbuildings, but was repelled after McGrew intentionally shelled one to start a fire and the Dakota burned the other before retreating.

[20] Dakota attempts to set fire to the rest of the fort with flaming arrows failed, as the shingles were still damp from the previous day's rains.

[20] Nearly out of ammunition, the garrison resorted to firing 3/4" sections of iron bars from the blacksmith shop that had been cut to size under the direction of Mrs. Muller.

Lt. Sheehan was forced to order the buildings on that side to be set ablaze to stop the Indians sneaking into the Fort through them.