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

Union forces led by General Ulysses S. Grant (in his first combat leadership role) overrun a Confederate camp  in Belmont, Missouri, on November 7, 1861, but are forced to retreat when Confederate reinforcements arrive.

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.

General Leonidas Polk's invasion of Columbus ends Kentucky's policy of neutrality and turns it against the Confederacy.

Grant uses river transport and Andrew Hull Foote's gunboats of the Western Flotilla to threaten the Confederacy's "Gibraltar of the West" at Columbus, Kentucky.

Although rebuffed at Belmont, Grant cuts off Columbus.

The Confederates, lacking their own gunboats, are forced to retreat and the Union takes control of western Kentucky and opens Tennessee in March 1862.

The Union military command in the West, however, suffers from a lack of unified command, organized by November into three separate departments: the Department of Kansas, under Major General David Hunter, the Department of the Missouri, under Major General Henry Halleck, and the Department of the Ohio, under Brigadier General Don Carlos Buell (who had replaced Brigadier General William Tecumseh Sherman).

By January 1862, this disunity of command is apparent because no strategy for operations in the Western theater can be agreed upon.

Buell, under political pressure to invade and hold pro-Union East Tennessee, moves slowly in the direction of Nashville, but achieves nothing more substantial toward his goal than minor victories at Middle Creek (January 10, 1862) under Colonel James A. Garfield and Mill Springs (January 19) under Brig. Gen. George Henry Thomas. (Mill Springs is a significant victory in a strategic sense because it breaks the end of the Confederate Western defensive line and opens the Cumberland Gap to East Tennessee, but it gets Buell no closer to Nashville.)

Kentucky's alternative government is recognized by the Confederate government, which had admitted Kentucky into the Confederacy in December 1861.

Using the rail system resources of the Mobile and Ohio Railroad, General Leonidas Polk has been able to quickly fortify and equip the Confederate base at Columbus.

General Albert Sidney Johnston's forward defense is broken.

As Grant had anticipated, Polk's position at Columbus is untenable, and he withdraws soon after Donelson falls.

Grant has also cut the Memphis and Ohio Railroad that previously had allowed Confederate forces to move laterally in support of each other.

General P.G.T. Beauregard had arrived from the East to report to Johnston in February, and he commands all Confederate forces between the Mississippi and Tennessee Rivers, which effectively divides the unity of command so that Johnston controls only a small force at Murfreesboro, Tennessee.