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Belmont (1861), Forts Henry and Donelson (1862)

On November 2, 1861, Lincoln removed Frémont from command, freeing Grant to attack Confederate soldiers encamped in Cape Girardeau, Missouri. On November 5, Grant, along with Brigadier General John A. McClernand, landed 2,500 men at Hunter's Point, and on November 7 engaged the Confederates at the Battle of Belmont. The Union army took the camp, but the reinforced Confederates under Brigadier Generals Frank Cheatham and Gideon J. Pillow forced a chaotic Union retreat. Grant had wanted to destroy Confederate strongholds at Belmont, Missouri, and Columbus, Kentucky, but was not given enough troops and was only able to disrupt their positions. Grant's troops escaped back to Cairo under fire from the fortified stronghold at Columbus. Although Grant and his army retreated, the battle gave his volunteers much-needed confidence and experience.

Columbus blocked Union access to the lower Mississippi. Grant and lieutenant colonel James B. McPherson planned to bypass Columbus and move against Fort Henry on the Tennessee River. They would then march east to Fort Donelson on the Cumberland River, with the aid of gunboats, opening both rivers and allowing the Union access further south. Grant presented his plan to Henry Halleck, his new commander in the newly created Department of Missouri. Halleck rebuffed Grant, believing he needed twice the number of troops. However, after consulting McClellan, he finally agreed on the condition that the attack would be in close cooperation with the navy Flag Officer, Andrew H. Foote. Foote's gunboats bombarded Fort Henry, leading to its surrender on February 6, 1862, before Grant's infantry even arrived.

Grant ordered an immediate assault on Fort Donelson, which dominated the Cumberland River. Unaware of the garrison's strength, Grant, McClernand, and Smith positioned their divisions around the fort. The next day McClernand and Smith independently launched probing attacks on apparent weak spots but were forced to retreat. On February 14, Foote's gunboats began bombarding the fort, only to be repulsed by its heavy guns. The next day, Pillow attacked and routed McClernand's division. Union reinforcements arrived, giving Grant a total force of over 40,000 men. Grant was with Foote four miles away when the Confederates attacked. Hearing the battle, Grant rode back and rallied his troop commanders, riding over seven miles of freezing roads and trenches, exchanging reports. When Grant blocked the Nashville Road, the Confederates retreated back into Fort Donelson. On February 16, Foote resumed his bombardment, signaling a general attack. Confederate generals John B. Floyd and Pillow fled, leaving the fort in command of Simon Bolivar Buckner, who submitted to Grant's demand for "unconditional and immediate surrender".

Grant had won the first major victory for the Union, capturing Floyd's entire army of more than 12,000. Halleck was angry that Grant had acted without his authorization and complained to McClellan, accusing Grant of "neglect and inefficiency". On March 3, Halleck sent a telegram to Washington complaining that he had no communication with Grant for a week. Three days later, Halleck claimed "word has just reached me that... Grant has resumed his bad habits (of drinking)." Lincoln, regardless, promoted Grant to major general of volunteers and the Northern press treated Grant as a hero. Playing off his initials, they took to calling him "Unconditional Surrender Grant".