Allan Nevins argued that the Civil War was an "irrepressible" conflict, adopting a phrase from Senator William H. Seward. Nevins synthesized contending accounts emphasizing moral, cultural, social, ideological, political, and economic issues. In doing so, he brought the historical discussion back to an emphasis on social and cultural factors. Nevins pointed out that the North and the South were rapidly becoming two different peoples, a point made also by historian Avery Craven. At the root of these cultural differences was the problem of slavery, but fundamental assumptions, tastes, and cultural aims of the regions were diverging in other ways as well. More specifically, the North was rapidly modernizing in a manner some perceived as threatening to the South. Historian McPherson explains: