Despite his misgivings about its defensibility, Johnston agreed to Beauregard's advice that he should reinforce Fort Donelson with another 12,000 men, knowing that a defeat there would mean the inevitable loss of Middle Tennessee and the vital manufacturing and arsenal city of Nashville.

[7] Johnston wanted to give command of Fort Donelson to Beauregard, who had performed ably at Bull Run, but the latter declined because of a throat ailment.

Early in the morning of February 11, Grant held a council of war in which all of his generals supported his plans for an attack on Fort Donelson, with the exception of Brig.

[14] Floyd's Confederate force of approximately 17,089 men consisted of three divisions (Army of Central Kentucky), garrison troops, and attached cavalry.

With the arrival of Grant's army, General Johnston ordered Floyd to take any troops remaining in Clarksville to aid in the defense of Fort Donelson.

[21] On February 13, several small probing attacks were carried out against the Confederate defenses, essentially ignoring orders from Grant that no general engagement be provoked.

[22] General Grant had Commander Henry Walke bring the Carondelet up the Cumberland River to create a diversion by opening fire on the fort.

[24] Although the weather had been mostly rainy up to this point in the campaign, a snowstorm arrived the night of February 13, with strong winds that brought temperatures down to 10–12 °F (−12 °C) and deposited 3 inches (8 cm) of snow by morning.

Because of the proximity of the enemy lines and the active sharpshooters, the soldiers could not light campfires for warmth or cooking, and both sides were miserable that night, many having arrived without blankets or overcoats.

This provided sufficient troops to extend McClernand's right flank to be anchored on Lick Creek, by moving Col. John McArthur's brigade of Smith's division from the reserve to a position from which they intended to plug the 400 yards (370 m) gap at dawn the next morning.

Grant realized that any success at Fort Donelson would have to be carried by the army without strong naval support, and he wired Halleck that he might have to resort to a siege.

[29] Despite their unexpected naval success, the Confederate generals were still skeptical about their chances in the fort and held another late-night council of war, where they decided to retry their aborted escape plan.

At dawn on February 15, the Confederates launched an assault led by Pillow against McClernand's division on the unprotected right flank of the Union line.

[30] The Confederate plan was for Pillow to push McClernand away and take control of Wynn's Ferry and Forge Roads, the main routes to Nashville.

The attack started well, and after two hours of heavy fighting, Pillow's men pushed McClernand's line back and opened the escape route.

By 9:30 a.m., as the lead Union brigades were falling back, Nathan Bedford Forrest urged Bushrod Johnson to launch an all-out attack on the disorganized troops.

[36] The Confederate offensive ended around 12:30 p.m., when Wallace's and Thayer's Union troops formed a defensive line on a ridge astride Wynn's Ferry Road.

As Grant rode back from the river, he heard the sounds of guns and sent word to Foote to begin a demonstration of naval gunfire, assuming that his troops would be demoralized and could use the encouragement.

Grant observed that some of the Confederates (Buckner's) were fighting with knapsacks filled with three days of rations, which implied to him that they were attempting to escape, not pressing for a combat victory.

Floyd also believed that C. F. Smith's division was being heavily reinforced, so the entire Confederate force was ordered back inside the lines of Fort Donelson, giving up the ground they gained earlier that day.

Smith's two-brigade attack quickly seized the outer line of entrenchments on the Confederate right from the 30th Tennessee, commanded by Col. John W. Head, who had been left behind from Buckner's division.

[41] In the meantime, on the Union right, Lew Wallace formed an attacking column with three brigades—one from his own division, one from McClernand's, and one from Smith's—to try to regain control of ground lost in the battle that morning.

Smith, the 8th Missouri, and the 11th Indiana advanced a short distance up the hill using Zouave tactics, where the men repeatedly rushed and then fell to the ground in a prone position.

Smith attacked again, he could only hold for thirty minutes, and he estimated that the cost of defending the fort would be as high as a seventy-five percent casualty rate.

Forrest's horsemen rode toward Nashville through the shallow, icy waters of Lick Creek, encountering no enemy and confirming that many more could have escaped by the same route, if Buckner had not posted guards to prevent any such attempts.

Smith was now in a good position to move on the fort, having captured the outer lines of its fortifications, and was under orders to launch an attack with the support of other divisions the following day.

The surrender was a personal humiliation for Buckner and a strategic defeat for the Confederacy, which lost more than 12,000 men, 48 artillery pieces, much of their equipment, and control of the Cumberland River, which led to the evacuation of Nashville.

The capture of Forts Henry and Donelson were the first significant Union victories in the war and opened two great rivers to invasion in the heartland of the South.

Grant had written in his memoirs that if all Union troops in the west were unified under a single commander to capitalize on the victory at Fort Donelson, the fighting in the western theaters of the civil war would've been closed quickly.

Johnston evacuated Nashville on February 23, surrendering this important industrial center to the Union and making it the first Confederate state capital to fall.