Missouri, like Arkansas, had a large pro-Union population and a large pro-Confederacy group as well. In March of 1861, both Arkansas and Missouri voted NOT to secede. Arkansas then voted to join the Confederacy in May of 1861 and the Missouri legislature voted to do the same in October. The bulk of Missouri leaders had supported secession from the beginning but a lot of political shenanigans and maneuverings postponed the action.
The Federal government could ill-afford to lose Missouri to the Confederacy for multiple reasons, not the least of which was a huge arsenal of weapons at St. Louis. The state of Missouri also offered control of both the Missouri and Mississippi Rivers, crucial routes for military supply.
Missouri and Arkansas share another Civil War similarity. Shortly after Arkansas voted not to secede, President Lincoln sent a letter to the Arkansas governor requiring that Arkansas furnish troops to the Union Army. Arkansas's response was voting to secede. Governor Jackson of Missouri received a similar letter from Lincoln. His response, "not a man would the State of Missouri furnish to carry on such an unholy crusade."
Because of that, the Union Army moved quickly into Missouri. With no Confederate forces, Missoui was served only by General Sterling Price's Missouri Guard. And the war was on!