Military and political situation
At the beginning of the American Civil War, Missouri declared that it would be an "armed neutral" in the conflict, and not send materials or men to either side. On April 20, 1861, a secessionist mob seized the Liberty Arsenal, increasing Union concerns in the state. The neutrality was put to a major test on May 10, 1861, in what became known as the Camp Jackson Affair. Governor Claiborne F. Jackson had called out the Missouri Volunteer Militia (MVM) to drill on the edge of St. Louis in Lindell Grove. The governor had clandestinely obtained artillery from the Confederacy and smuggled it into the militia encampment – referred to as "Camp Jackson". Capt. Nathaniel Lyon was aware of this shipment and was concerned the militia would move on the St. Louis Arsenal. Thomas W. Sweeny was put in command of the arsenal's defense, and Lyon surrounded the militia camp with Union troops and home guards, forcing the surrender of the militia. When he marched the prisoners through the streets to the arsenal, some angry members of the crowd began to press against the procession. Taunts and jostling eventually led to gunfire and many deaths, mostly civilians but also several soldiers and members of the militia.
A day later, the Missouri General Assembly created the Missouri State Guard (replacing the MVM) theoretically to defend the state from attacks from perceived enemies from either side of the war (although from its inception, all Missouri State Guard planning focused on conflict with Federal forces). The governor appointed Sterling Price as the commander with the rank of major general of state forces. The state guard was divided into divisions, with each division consisting of units raised from a military district of Missouri and command by a brigadier general. Because much of their recruiting areas were behind Union lines, many divisions were the size of a brigade, consisting of only a few regiments. Fearing Missouri's tilt to the South, William S. Harney, the Federal commander of the U.S. Army's Department of the West (which included Missouri) negotiated the Price-Harney Truce on May 12, 1861, which nominally agreed to cooperation between the U.S. Army and the MSG to maintain order in Missouri and protect it from outside interference. Jackson publicly declared his support for the truce, while secretly requesting that Confederate forces enter Missouri to "liberate" Missouri from Federal control. After complaints by Missouri Unionists, Harney was replaced by Lyon (who was promoted to brigadier general of volunteers), further undermining the fragile truce. On June 12, 1861, Lyon and Jackson met at the St. Louis' Planter's House Hotel in a last attempt to avoid a resumption of fighting. Both sides were inflexible, with Lyon demanding the right to inspect any area of the state for Confederate intervention, and Jackson refusing and demanding that Federal forces be restricted to the St. Louis metropolitan area. The meeting ended with Lyon saying:
This means war. In an hour one of my officers will call for you and conduct you out of my lines.
Lyon sent a force under Sweeney to Springfield while his own forces quickly captured the capital and pursued Jackson, Price, and the now-exiled state government across Missouri. Skirmishes followed, including the Battle of Boonville on June 17 and the Battle of Carthage on July 5. In light of the crisis, the delegates of the Missouri Constitutional Convention that had rejected secession in February reconvened. On July 27, the convention declared the governor's office vacant and selected Hamilton Rowan Gamble to be the new provisional governor.
By July 13, 1861, Lyon's army of approximately 6,000 men was encamped at Springfield. His force was composed of the 1st, 2nd, 3rd, and 5th Missouri infantry, the 1st Iowa Infantry, the 1st Kansas and 2nd Kansas (infantry, several companies of regular army infantry and cavalry, and three batteries of artillery). He divided the units into four brigades commanded by Major Samuel D. Sturgis, Colonel Franz Sigel, Lieutenant Colonel George Andrews, and Colonel George Dietzler.
By the end of July 1861, the Missouri State Guard was camped about 75 mi (121 km) southwest of Springfield and had been reinforced by Confederate Brigadier General Benjamin McCulloch and Arkansas state militia Brigadier General N. Bart Pearce, making the mixed Missouri/Arkansas/Confederate force over 12,000 strong. They developed plans to attack Springfield, but Lyon marched out of the city on August 1 in an attempt to surprise the Southern forces. The armies' vanguards skirmished at Dug Springs, Missouri on August 2. The Union force emerged as the victor, but Lyon learned he was outnumbered more than two-to-one and retreated back to Springfield. McCulloch, now in command of the Missourian army, gave chase. By August 6, his force was encamped at Wilson's Creek, 10 miles (16 km) southwest of the city. Price favored an immediate attack on Springfield but McCulloch, doubtful about the quality of the Missouri State Guard, preferred to remain in place. After Price threatened to launch an attack without his support, McCulloch agreed to an attack at dawn on the 10th but when a rainstorm started during the evening of the ninth, he cancelled his plans and ordered his troops back to camp.
Outnumbered, Lyon planned to withdraw northeast to Rolla to reinforce and resupply, but not before launching a surprise attack on the Missourian camp to delay pursuit. Sigel proposed striking McCullough in a pincer movement, which would split the already outnumbered Union force; he planned to lead 1,200 men in a flanking maneuver while the main body under Lyon struck from the north. Lyon concurred, and in accord with Sigel's plan, the Union army marched out of Springfield on the rainy night of August 9, 1861, leaving about 1,000 men to protect supplies and cover the retreat.