Pergrin was called to active duty in April 1941, with the rank of 2nd Lieutenant. After helping train numerous engineering platoons and companies, he was transferred to the new 291st Engineer Combat Battalion in April, 1943 to assist in its training. In August, he was given command of the battalion when it completed its states-side training. During training in the United States and Britain, then Major, later Lt. Colonel, Pergrin emphasized individual leadership which he credited as the main reason for the unit's success in completing its missions, especially while under enemy fire.