While billeted at Stockton, in December 1861, Roberts faced a court-martial for, among other charges, shaking a German-American civilian named William Beck and calling him a "damned liar" and "infernal Dutch hound."

[1][6][4] On the night of April 1, 1862, Roberts fought in the Battle of Island Number Ten, in which he distinguished himself by leading fifty troops under cover of darkness to raid a Confederate shore battery and spike its guns.

[1][6] During the summer and fall of 1862, the 42nd Illinois participated in numerous skirmishes and engagements, including the Siege of Corinth and a sharp fight at Farmington, Mississippi.

As the Confederates pushed back the Union army, Roberts, on horseback, personally led a bayonet charge that stabilized the line, giving Sheridan time to regroup.

He was the last of Sheridan's three brigade commanders to die at Stones River—Colonel Frederick Schaefer and Brigadier General Joshua W. Sill had been killed earlier that day.

He was young, tall, handsome, brave, and dashing and possessed a balanced wheel of such good judgment that, in his sphere of action, no occasion could arise, from which he would not reap the best results.