The focus early in the war is on two critical states: Missouri and Kentucky.

The loss of either will be a crippling blow to the Union cause.

Primarily because of the successes of Captain Nathaniel Lyon and his victory at Boonville in June, Missouri is held in the Union.

Confederate General Leonidas Polk invades neutral Kentucky on September 3, 1861, prompting the state legislature to ask for Union assistance.

The state of Kentucky, with a pro-Confederate governor and a pro-Union legislature, has declared neutrality between the opposing sides.

This neutrality is first violated on September 3, when Confederate Major General Leonidas Polk occupies Columbus, considered key to controlling the Lower Mississippi.

Union Brigadier General Ulysses S. Grant, displaying the personal initiative that will characterize his later career, bloodlessly seizes Paducah two days later, which gives the Union control of the mouth of the Tennessee River.

Henceforth, neither adversary will respect the proclaimed neutrality of the state; while most of the state government will remain loyal to the Union, the pro-Confederate elements of the legislature will organize a separate government in Russellville that will be admitted into the Confederate States in December.

This sequence of events is considered a victory for the Union because Kentucky will never formally side with the Confederacy, and if the Union had been prevented from maneuvering within Kentucky, its later successful campaigns in Tennessee would have been more difficult

Four slave states—Delaware, ...

...Missouri, ...

...Maryland, and ...

...Kentucky—after prolonged contests in their legislatures, had voted by September 11 to remain in the Union.

On September 18, the Kentucky Legislature takes steps to oust Confederate forces.

On the Confederate side, General Albert Sidney Johnston commands all forces from Arkansas to the Cumberland Gap.

He is faced with the problem of defending a broad front with numerically inferior forces, but he has an excellent system of lateral communications, permitting him to move troops rapidly where they are needed, and he has two able subordinates, Polk and Major General William J. Hardee.

Johnston also gains political support from secessionists in central and western counties of Kentucky via a new Confederate capital at Bowling Green, set up by the Russellville Convention.

The Western theater in the Civil War refers to military operations between the Appalachian Mountains and the Mississippi River, including the states of Alabama, Georgia, Florida, Mississippi, North Carolina, Kentucky, South Carolina and Tennessee, as well as parts of Louisiana.