Immediately after the Civil War began, Heckman enlisted in the 1st Pennsylvania Volunteer Regiment and was elected captain.

General Heckman led a brigade in Foster's expedition at the battles of Kinston, White Hall and Goldsborough.

Heckman led his division at the battle of Chaffin's Farm in support of General George J. Stannard's main attack against Fort Harrison.

Instead of coming directly to the aid of Stannard's division, Heckman's men launched a costly attack against Confederate trenches near Forts Gilmer and Johnson.

[2] During the fighting in Fort Harrison, XVIII Corps commander Edward O. C. Ord was severely wounded and carried from the field.

With this change in command at a crucial point the fighting ground to a halt and the Union forces constructed a defensive line.