I think it was wise of the producers of the film to pick a key moment in the great and bloody saga of the Civil War and focus on it. The key part is the battle in the House of Representitives in early 1865 for passage of the 13th Amendment. The amendment will abolish slavery forever, if approved by 2/3rds of the loyalist state legislatures.
Time is running out because the war is ending and Lincoln's fear seems to be that those states that will return to the Union under force will try and wittle away any other anti-slavery measures through the courts or the reconstituted Congress to undo the fulcrum reason for the war and restore chattel slavery for 3-4 million souls.
There are no great battle scenes (although there is a bloody skirmish between Blue and Grey units) and, for Spielberg at least, very little in the way of epic sweep. That is all to to the good--the focus here should be on the interpersonal relations of Abraham Lincoln with his Cabinet; the soldiers he meets and talks to near battlefields; with the wounded and badly maimed men in army hospitals, and with his own family, especially his son Robert, a grown man who wants to be in uniform just as much as his mother Mary Todd Lincoln (who has already lost one son to disease three year earlier) wants to keep him out.
No wonder Lincoln went to the theatricals around Washington so often, twice or three times a week it is said. He also read humorous books by American comic writers like Artemus Ward and told old hoary but often proverbial stories of his days as a "circuit-riding" country lawyer in rural Illinois. There seems little relief for a man whose very face changed from young middle-age to old as Moses in just four years in office.
This subplot brings us more of Lincoln as a private man, one who finds little comfort at home with a hysterical wife and the memories of his dead son Willie, the one many historians say was the most like his father. What little solace he gets is from his youngest boy, Tad, a youngster who looks at photos of slave children in glass photo cells, reminding the viewer of the horrible price America has to pay to undo 250 years of slavery on her shores. The matter is still in suspense at this point. Lincoln has technically freed the slaves in the rebel states , but he knows it must be codified in the Constitution.
This is the central drive of the film--the American Establishment finally living up to the ideals of its founding document, the Declaration of Independence and the notion of men being born free and equal before the law.
Spielberg and the writer Tony Kushner have given us a Lincoln of flash and bone, one who swears, gets angry, tells funny stories and walks the halls of the Executive Mansion unable to sleep with the cares of his office burdening him. Hundreds of thousands of men have died and been maimed and whole cities burned to the ground in this terrible war. Can something of meaning be gained from all of it?
All the main actors, especially Daniel Day-Lewis, Sally Field and Tommy Lee Jones (as the "Radical" Republican Thaddeus Stephens ) are wonderful in the film. And anyone interested in how freedom for slaves was really accomplished (by all the tricks and carrots and sticks men of power can use at their disposal, as well as the sword) will be satisfied by this film.