Historians call the period from the 1840s to the end of Reconstruction (1877) the Civil War Era. The American Civil War itself lasted from 1861-1865. The Mexican American War, fought between 1846 and 1848, ended with the largest acquisition of territory since the Lousiana Purchase in 1804, what is today the southwestern states of New Mexico, Arizona, and California. The long standing argument over the compatibility of slavery with the American conception of freedom and equlity ("All men are created equal") saw its apex in the future status of these territories and in the western half of the continent generally. In the power struggle between the southern slave states and the northern industrialized states can be found the direct causes of the American Civil War.