In a career that spanned more than forty years, Buchanan held numerous commands (including several forts) and received multiple citations for bravery and distinguished service.

Buchanan received his appointment to United States Military Academy at West Point during Adams' administration and graduated in 1830.

His assignments included service in the Black Hawk War (he commanded gunboats during the Battle of Bad Axe) and against the Seminoles, as well as in the removal of the Cherokees to the Indian Territory.

In 1847 Buchanan became a veteran member of the Aztec Club of 1847 – a military society of officers who had served in the Mexican War.

[5] Buchanan, by then nicknamed "Old Buck" by his men, commanded the 1st Brigade, 2nd Division, V Corps during the Maryland Campaign (part of Brigadier General George Sykes's Regulars).

At Antietam, Buchanan strongly protested a decision to halt his advance on what he maintained was a weakly defended portion of the enemy line.

In his opinion, his Regulars could have and should have carried Cemetery Hill, defended primarily by artillery with only the depleted Virginia brigade of Richard B. Garnett in support.

[3] Historian Ezra Warner suggests that Buchanan's association with Fitz John Porter was the reason for the Senate's inaction on the nomination.

[4] Buchanan then went on recruiting duty and two months later was placed in command of the defenses of Fort Delaware, a prisoner of war facility, March–April, 1863.

[10] After the war, as colonel in the Regular Army, Buchanan was placed in command of the 1st U.S. Infantry at New Orleans and helped enforce Reconstruction activities with his men.