I te tīmatanga o te Pakanga ā-Iwi, ka uru anō a Hays ki te mahi hei kaikoroneli o te 63rd Pennsylvania Volunteer Infantry Regiment, e mau ana hoki i te tūranga rangatira i te 16th U.S. Infantry i roto i te ope taua i te ra o Mei 14, 1861. I mohiotia e ana tangata ko ia "he toa ano he raiona". I te pakanga o te Peninsula, i whakauruhia ia me tana roopu ki te piriti tuatahi o te wehenga a Kearny o Heintzelman's III Corps. I whawhai ia i nga pakanga o Yorktown, Williamsburg, Seven Pines, Savage's Station
, me Malvern Hill. I te mutunga o nga Pakanga o nga Ra e whitu, i tohua ia hei koroneela tuarua i roto i te ope taua mo te maia i roto i te mahi, na te mea i arahi a Hays i te whakapae bayonet me tana rerenga ki nga raina hoariri hei hipoki i te wehenga o tana pereti. I haere a Hays mo te wa poto i te mate mate i te marama i muri mai, i te mamae o te matapo me te paraire o tona ringa maui, nga whara i puta mai i te pakanga.