Lincoln wanted to delay the Confederate Peace Commissioners' peace offer and keep them out of D.C.
The reason for that is, I think, that conservative Republican congressman knowing that peace was nigh upon them wouldn't then have supported passage of the Amendment.
And that's because save
Which is to say, again, it was thought passage of the Amendment would hasten the end of the war.
That's what I'm not getting: why from the standpoint of the Confederacy would passage of the Amendment have caused it to end the war more quickly?
The only answer I can derive from the movie is that if that the end of slavery was a legal fait accomplit the Confederacy would no longer be motivated to keep fighting.
But this "answer" seems illogical, since the potential ending of slavery was a very cardinal thing the Confederacy was fighting against in the first place.