Columbia subject headings:
United States--History--1815-1861.
United States--History--Civil War, 1861-1865--Causes.
Harvard subject headings:
United States--History--1815-1861.
United States--History--Civil War, 1861-1865--Causes.
Princeton subject headings:
United States--History--1815-1861.
United States--History--Civil War, 1861-1865--Causes.