Philip Zelikow is a professor of history and an associate dean for the Graduate School of Arts and Sciences at the University of Virginia.

Having worked before at the intersection of Hollywood and history, helping a tiny bit with a respectable movie about the Cuban missile crisis called “Thirteen Days,” I approached the new movie “Lincoln” with measured expectations. I had seen how a film could immerse viewers in onscreen time travel without messing up the history too much. But that was the most I hoped for.

“Lincoln,” however, accomplishes a far more challenging objective: its speculations actually advance the way historians will consider this subject.

The movie, directed by Steven Spielberg and starring Daniel Day-Lewis as the 16th president, makes two especially interesting historical arguments.

The first is to explain why the passage of the 13th Amendment, which abolished slavery, was overwhelmingly important to Lincoln in January 1865....

The question has long vexed historians. The movie’s answer is that Lincoln and his right-hand man in this work, Secretary of State William H. Seward, realized that the war might end at any time and that, when it did, any prospect for passing the amendment as a means to win the war would end with it....

The movie goes on to argue that Lincoln calibrated the timing of this diplomacy, including delaying his own meeting with the Confederate envoys, to be sure that nothing from it would materialize in time to jeopardize passage of the amendment. Lincoln met with the envoys three days after the amendment had passed the Congress. The peace initiative came to nothing.

Historians have never been very clear on just why Lincoln entertained Blair’s project in the way he did. But there is, in fact, good evidence to support the filmmakers’ supposition of a deal linking the Blair peace initiative to the politics of the 13th Amendment....