Once war came, Fillmore supported Lincoln in his efforts to preserve the Union.[134] Fillmore commanded the Union Continentals, a corps of home guards of males over the age of 45 from Upstate New York. The Continentals trained to defend the Buffalo area in the event of a Confederate attack. They performed military drills and ceremonial functions at parades, funerals, and other events. The Union Continentals guarded Lincoln's funeral train in Buffalo. They continued operations after the war, and Fillmore remained active with them almost until his death.[135][136]
Despite Fillmore's zeal in the war effort, he gave a speech in early 1864 calling for magnanimity towards the South after the war and counted its heavy cost, both in finances and in blood. The Lincoln administration saw the speech as an attack on it that could not be tolerated in an election year, and Fillmore was criticized in many newspapers and was called a Copperhead and even a traitor. That led to lasting ill-feeling against Fillmore in many circles.