He later became a noted specialist on the American Civil War era, as well as southern social and economic history.
They traced numerous ways in which the Celtic culture shaped social, economic and military behavior.
According to their thesis, the South lost the Civil War because Southerners fought like their Celtic ancestors, who were very fierce fighters and intensely loyal to their leaders but lacked efficiency, perseverance, and foresight.
McWhiney continued exploring the thesis in Cracker Culture: Celtim Folkways in the Old South {1988), in which he extensively explored fundamental similarities between behaviors in the Old South and those in pre-modern Ireland, Wales, Scotland, and other areas in Great Britain where Celtic peoples settled.
Some of the fundamental attributes that caused the Old South to adopt anti-English values and practices were Celtic social organization, language, and means of livelihood.
According to the thesis, it was the Celtic values and traditions that set the agrarian South apart from the industrialized civilization developing in the North.
It did receive some verification in the work of historian David Hackett Fischer in Albion's Seed: Four British Folkways in America published in 1989.