I te tau 1860, ka tae a Parnell ki te United States ka uru atu ki te US Army i Brooklyn, New York i te timatanga o te American Civil War i te tau e whai ake nei. I noho ia hei mema o te 4th Regiment New York Volunteer Cavalry, e mohiotia whanuitia ana ko "Dickel's Mounted Rifles", e mahi ana me te Army of the Potomac. I nga tau tuatahi e rua o te pakanga, i uru ia ki nga kaupapa a Colonel Louis Blenker i te Raorao o Shenandoah me West Virginia. I kite hoki ia i nga pakanga o Cross Keys, Port Republic, Cedar Mountain, me te Pakanga Tuarua o Bull Run. I te taha o te Cavalry Corps, i whawhai ano ia ki nga pakanga o Fredericksburg, Beverly Ford, Brandy Station, Stoneman's Raid, Aldie, me Middleburg. I te 21 o Pipiri 1863, i hopukina ia i muri i tana arahi i te whakapae hoiho i te Pakanga o Upperville, a, i mau poto ia hei herehere-a-whawhai. I mawhiti ia e rua marama i muri mai ka haere ki Petersburg, West Virginia i reira ia i hui tahi ai me tana pu. I muri mai ka kite ia i nga pakanga o te Wilderness, Spotsylvania, Trevilian Station, Petersburg, Lee's Mills, Winchester
me Cedar Creek i roto i ētahi atu mahi.