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.)

General Don Carlos 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 Brigadier General 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.)

The one clear Confederate victory in the West is the Battle of Chickamauga.

After William Rosecrans' successful Tullahoma Campaign, Bragg, reinforced by Lieutenant General James Longstreet's corps (from Lee's army in the east), defeats Rosecrans, despite the heroic defensive stand of Major General George Henry Thomas.

Rosecrans has established his army at Chickamauga, Georgia, twelve miles (nineteen kilometers) southeast of Chattanooga, the location of a vital rail center in the struggle over Tennessee.

Bragg collects reinforcements and prepares to do battle, assisted by General James Longstreet, on September 19, 1863.

The main body of the Union army, dazed by the ferocious Confederate assault, gives way and retreats in disorder after two days of raging conflict in a tangled forest along Chickamauga Creek.

Union General George H. Thomas, the “Rock of Chickamauga”, skillfully organizes the defenses and withstands the attack until the assistance of a reserve corps makes possible an orderly withdrawal to Chattanooga.

The defeat of the fifty-eight thousand two hundred and twenty two Union troops by sixty six thousand three hundred and twenty-six Confederates is one of the bloodiest engagements of the war, Rosecrans losing sixteen thousand one hundred and seventy troops in his retreat.

Total Union casualties include sixteen hundred and fifty seven killed, nine thousand seven hundred and fifty six wounded, and four thousand seven hundred and fifty-seven missing.

Confederate losses list two thousand three hundred and twelve killed, fourteen thousand six hundred and seventy-four wounded, and fourteen hundred and sixty-eight missing, totaling eighteen thousand four hundred and fifty four casualties.

Both Grant and Sherman initially had objectives to engage with and destroy the two principal armies of the Confederacy, relegating the capture of important enemy cities to a secondary, supporting role.

This is a strategy that President Abraham Lincoln has emphasized throughout the war, but Grant is the first general who actively cooperates with it.

As their campaigns progress, however, the political importance of the cities of Richmond and Atlanta begins to dominate their strategy.

With Chattanooga and Vicksburg firmly under the control of the North by the end of 1863, Atlanta is now the logical point for Union forces to attack in their western campaign.

Distant from earlier fighting, Atlanta has become an important Confederate railroad, supply, and manufacturing center and a gateway to the lower South.

Atlanta has become a critical target.

The city of twenty thousand had been founded at the intersection of four important railroad lines that supply the Confederacy and is a military manufacturing arsenal in its own right.

Atlanta's nickname of "Gate City of the South" is apt—its capture would open virtually the entire Deep South to Union conquest.

Grant's orders to Sherman are to "move against Johnston's Army, to break it up and to get into the interior of the enemy's country as far as you can, inflicting all the damage you can against their War resources."

Grant begins his advance into Virginia while Sherman, a red-haired man who will gain a lasting reputation as a major architect of modern warfare, leaves Chattanooga with one hundred thousand men to begin his march through Georgia on May 5-7.

Sherman's force is composed of three subordinate armies: the Army of the Tennessee (Grant's and later Sherman's army of 1862–63) under Major General James B. McPherson; the Army of the Cumberland under Major General George H. Thomas; and the relatively small Army of the Ohio (composed of only the XXIII Corps) under Major General John M. Schofield.

Their principal opponent is the Confederate Army of Tennessee, commanded by Gen. Joseph E. Johnston, who had replaced the unpopular Braxton Bragg after his defeat in Chattanooga in November 1863.

The fifty thousand-man army consists of the infantry corps of Lieutenant Generals William J. Hardee, John Bell Hood, and Leonidas Polk, and a cavalry corps under Major General Joseph Wheeler.

Sherman launches demonstration attacks against Johnston's position on the long, high mountain named Rocky Face Ridge while McPherson's Army of the Tennessee advances stealthily around Johnston's left flank toward the town of Resaca and Johnston's supply line on the Western & Atlantic Railroad.

Unfortunately for Sherman, McPherson encounters a small Confederate force entrenched in the outskirts of Resaca and cautiously pulls back to Snake Creek Gap, squandering the opportunity to trap the Confederate army.

As Sherman swings his entire army in the direction of Resaca, Johnston retires to take up positions here.

The Confederate government had granted General Joseph E. Johnston's request for reinforcements to his camps around Dalton, Georgia, in early May 1864.

As the brigade of Brigadier General James Cantey had started to move through the city on May 7, cavalry scouts had alerted Johnston that a large number of Union troops were moving towards Rome, Georgia, on roads that led through Resaca.

During the remainder of May 7 and the day of May 8 Cantey's brigade has had time to entrench and set up defenses.

On May 9, the Army of the Tennessee, under the command of James B. McPherson, had moved out of Snake Creek Gap and immediately run into a Confederate cavalry brigade ordered to scout the area the day before under the command of Colonel Warren Grigsby.

After a fierce battle, Brigadier General Thomas W. Sweeny had formed a defensive line, driving the Confederates back to Resaca, several miles to the east.

Sherman's plan, as written in his memoirs, is to hold the railroad and telegraph lines south of Dalton, so that Johnston will either evacuate his position at Dalton or detach a section of his army to fight Sherman on a ground that has more of an advantage to Sherman.

He has devoted the Army of the Tennessee to this mission, while the Army of the Cumberland and the Army of the Ohio, commanded by George H. Thomas and John M. Schofield, respectively, will feign attacks in the Confederates' front.

As McPherson's two Corps leave the woods, they skirmish with Confederate cavalry for a while until the cavalry id able to withdraw to a line of fortifications on the outer edge of the city, where they are reinforced by the 37th Mississippi, a regiment in James Cantey's brigade.

In the evening, Sherman sends his only cavalry, the 9th Illinois Mounted Infantry, northeast to scout out the best route to the Western & Atlantic Railroad.

Meanwhile, skirmishers in Major General Grenville Dodge's XVI Corps move to attack a line of fortifications along Camp Creek, held by Confederate cavalry, the remainders of Cantey's brigade, two twelve pound Napoleonic-era batteries and a fresh brigade under Confederate brigadier general Daniel H. Reynolds, which is the lead of the column of twenty thousand men sent out from Atlanta by John Bell Hood.

Johnston had withdrawn his forces from Rocky Face Ridge to the hills around Resaca.

On May 13, the Union troops had tested the Rebel lines to pinpoint their whereabouts.

The next day, full scale fighting had occurred, and the Union troops had been generally repulsed except on the Rebel right flank, where Sherman had not fully exploited his advantage.

On May 15, the battle continues with no advantage to either side until Sherman sends a force across the Oostanaula River, at Lay's Ferry, using newly delivered Cumberland pontoon bridges and advances towards Johnston's railroad supply line.

Johnston, unable to halt the Union turning movement caused by Sherman's crossing of the Oostanaula, had been forced to retire, burning the railroad span and a nearby wagon bridge in the early morning of May 16.

After the Union repairs the bridges and transports more men over, they continue in the pursuit of the Confederates, leading to the Battle of Adairsville on May 17.

There are sixty-one hundred combined casualties: thirty-five hundred for the Union and twenty-six hundred for the Confederacy.