He was wounded in an artillery duel while his regiment was in reserve at the Battle of Belmont, and in April 1862 was captured when the Confederate defenses of Island Number Ten fell.

Imprisoned at Fort Warren for several months, Gantt returned to Arkansas but failed to receive another command appointment amid rumors of alcoholism and womanizing.

As an opponent of Arkansas's ruling political "Family", Gantt ran for a seat in the United States House of Representatives in 1860.

[6] Abraham Lincoln won the 1860 United States presidential election, and Gantt began canvassing northern and western Arkansas with secessionist speeches.

In late July, he was elected colonel of the 12th Arkansas Infantry Regiment;[1] Gantt had previously requested to be made a major general.

[2][10] In December, another regiment was added to Gantt's command, and he and his men were transferred to the defenses of the Island Number 10 and New Madrid, Missouri, area.

Believing that the Confederacy no longer offered him a chance at prominence, Gantt made his way to the Union lines at Vicksburg, Mississippi, and surrendered.

[17] Gantt spoke against the Confederacy, slavery, and secession,[15] and in 1863 and 1864 gave speeches in the northern United States designed to strengthen support in the Union for continuing the war.

[2] These speeches contained some militant rhetoric; Finley states that this indicates that he had "disengaged himself from the carnage of the battlefield", attributing Gantt's actions to a desire to "regain and maintain political power at all costs".

[20] In this role, he oversaw the relations between freed slaves and white Arkansans in his district;[2] he spent much time reviewing and mediating labor contracts.

Gantt also organized fundraising for a hospital, supported education for former slaves, and encouraged African Americans in his district to have formal marriages.

His work with the Bureau had made him unpopular with Arkansas's class of white elites, which would block his hopes for higher political office.

[2] Gantt opposed the activities of the Ku Klux Klan and in 1867 and 1868 supported Ulysses S. Grant's presidential election campaign.

[15] Finley notes that his work with Freedman's Bureau had a positive impact, and that he helped convince the Union that brutal peace terms in the Confederacy were not necessary.