The plantations that enslaved over three million black people were the principal source of wealth. Most were concentrated in "black belt" plantation areas (because few white families in the poor regions owned slaves). For decades, there had been widespread fear of slave revolts. During the war, extra men were assigned to "home guard" patrol duty and governors sought to keep militia units at home for protection. Historian William Barney reports, "no major slave revolts erupted during the Civil War." Nevertheless, slaves took the opportunity to enlarge their sphere of independence, and when union forces were nearby, many ran off to join them.