This question originally appeared on Quora. 
By Matthew Pinsker, Pohanka Chair for Civil War History, Dickinson College
The best way to answer this question is to begin by defining terms. When Lincoln wrote on August 22, 1862 in his famous open letter to Horace Greeley, "My paramount object in this struggle is to save the union and is not either to save or to destroy slavery," he was employing a word --"union" -- that meant different things to different people (and still does, by the way). For Lincoln and the Republicans, the union was never merely a collection of states. Nor was it a centralized federal government or some abstract attachment to a paper Constitution. This is the key point and what always leads to confusion. For Lincoln especially, the "union" was "the people" -- as in "We the people" and what should properly be considered the fundamental and most revolutionary American doctrine of popular sovereignty. Look carefully at all of Lincoln's wartime speeches and statements and you will see that behind the phrase "save the union," Lincoln always meant to protect the results of the 1860 election, which he believed had defined the popular will through a legitimate electoral process. That's how he justified calling himself a unionist even though he led a sectional party. That's why he refused practically all compromises during the secession crisis, because he believed that they failed to acknowledge how much the election mattered. And that's why he pursued increasingly "hard war" policies against the Confederacy, including emancipation, that ultimately turned the war into what he had once warned against, "a remorseless, revolutionary struggle." In other words, Lincoln was both anti-slavery and pro-union. In fact, he considered those positions one and the same, because he defined "union" as the popular will which by the 1860 election results had determined that the future of the country was to be free, or, as Lincoln put it at Gettysburg, to be "a nation conceived in liberty and dedicated to the proposition that all men are created equal." He was always willing to reunify the country on those terms and never willing to consider anything less. This is, by the way, exactly the question that Steven Spielberg's new movie, "Lincoln," intends to examine by focusing on the last few months of the war and what the movie-makers present as the fundamental choice that Lincoln navigated during that period between pushing for a Thirteenth Amendment to abolish slavery or pursuing potential peace talks with Confederates.