COREY M. BROOKS is assistant professor of history at York College of Pennsylvania. The essay was written for the History News Service. Contact: cbrooks4@ycp.edu

Like Abraham Lincoln’s Emancipation Proclamation, President Obama’s declaration that gay and lesbian Americans deserve a right to marry is a historic statement of principle, even though its practical policy implications are limited because the states, not the federal government, hold most of the power to define marriage.

Obama’s assertion that state-level prohibitions on same-sex marriage are morally objectionable but legally sound rings familiar to this Civil War historian. I can’t help but notice a close resemblance to the pre-Civil War views of millions of white Northerners (and perhaps more than a few white Southerners) on slavery.

Northerners understood that the Constitution protected slaveholding as a matter left to individual states, and for decades many had found this a comforting justification for their tolerance of an institution they knew should be intolerable. Slavery was a Southern problem, and as long as it could be kept a Southern problem, Northerners’ thinking went, they would bear little moral responsibility....