Halleck's headquarters were at St. Louis, and he restrained the Secessionists with a vigorous hand. Since the retrograde movement of Hunter, with Fremont's army, Price had been gathering a Confederate force in Missouri, and General John Pope was placed in command of a considerable body of troops to oppose him. Pope acted with great vigor and skill. He made a short, sharp, and effective campaign. Detachments from his army struck some blows here and there that were telling. One was inflicted by troops under General J. C. Davis, on the Blackwater, near Milford, which gave a stunning blow to the insurgents in that State. Davis found the enemy in a wooded bottom opposite his own forces. He carried a wellguarded bridge by storm, and struck the Confederates so hard that they fled in much confusion; and when they were closely pursued, they surrendered, in number about thirteen hundred, cavalry and infantry. The spoils of victory were eight hundred horses and mules, a thousand stand of arms, and over seventy wagons loaded with tents, baggage, ammunition, and supplies of every kind. In a brief space of time, the power of the insurgents in that quarter was paralyzed, and Halleck complimented Pope on his "brilliant campaign."

Pope had not only prevented organized troops from joining Price, but had compelled the latter to withdraw to the borders of Arkansas for supplies and safety. Feeling strengthened by Pope's success, Halleck prepared to put forth more vigorous efforts to suppress the insurrection. On the 3d of December he declared martial law in St. Louis; and, by a subsequent proclamation, he extended that system of rule to all railroads and their vicinities. Meanwhile, Price, relieved from immediate danger, and being promised reinforcements from Arkansas, moved back to Springfield, and there concentrated about twelve thousand men, halted his army, and prepared to spend the winter there. Halleck sent troops in that direction under General S. R. Curtis, assisted by Generals Davis, Sigel, Asboth, and Prentiss. They moved in three columns early in February (1862), when Price fled southward, and did not halt until he reached a good position in northern Arkansas. Curtis pursued him, and drove him further south; and Halleck was enabled to write to his Government, late in February, that he had "purged Missouri," and that the flag of the Union was "waving in triumph over the soil of Arkansas." The campaign in Missouri, for a few months, had been very active, beginning with Lyon's pursuit of the fugitive governor and his followers. From June, 1861, until late in February, 1862, there had been fought on Missouri soil sixty battles and skirmishes, with an aggregate loss on both sides, in killed, wounded and prisoners, of about twelve thousand men.

Curtis crossed the Arkansas line on the 18th of February in pursuit of Price, and had driven him and his followers over a range of hills known as the Boston Mountains. He then fell back and encamped in a strong position in the vicinity of Pea Ridge, a spur of the Ozark Mountains. In the meantime Price had been joined by General Earl Van Dorn, a dashing young officer, who was his senior in rank and now took the chief command. Forty heavy guns thundered a welcome. "Soldiers! " cried Van Dorn, in response, "behold your leader! He comes to show you the way to glory and immortal renown. He comes to hurl back the minions of the despots at Washington, whose ignorance, licentiousness, and brutality are equalled only by their craven natures. They come to free your slaves, lay waste your plantations, burn your villages, and abuse your loving wives and beautiful daughters."