They might not have been "allowed" but they did go on the battlefield. Sarah Edmonds wrote in her memoirs about helping wounded off the battlefield and having bullet holes in her dress. Also, Juliet Ann Opie Hopkins was wounded and left with a permanent limp at the battle of Sevens Pines. She received the injury while helping casualties off the field. Just two examples of nurses on the battlefield.
Originally Posted by "Doc" Nelson
Alexander's Battalion Field Hospital
"Yer prettier than a pair of red shoes with green laces". Wounded soldier to Phoebe Yates Pember, matron of Chimborazo Hospital.