Scene of Black Soldiers Talking with Lincoln was Annoyingly Unrealistic

In general this is a good movie with strong performances. It is closer to the historical record than many historical films. It's especially laudable that the film reveals how prominent racism was in the north as well as the south.

However, as others have noted, the battlefield scene in which African-American soldiers spoke with Lincoln in a non-deferential way was highly unrealistic. It would have strained credulity even to depict white soldiers behaving that way.

A movie can be unrealistic and great, but that depends on genre. This film was not supposed to be fantasy, sci-fi or magical realism, but a very realistic depiction of historical events around the adoption of the 13th Amendment. There is a difference between writing dialogue that didn't actually occur in the same words or creating composite characters in the interest of exposition and depicting a whole scene that was not just not factual but not even realistic.

Spielberg must have felt that in a movie about the abolition of slavery released in the 21st century it was problematic that all of the major characters were white. Unfortunately this reflected political reality at the time. Even in the north most African-Americans were disenfranchised and Congress was all-white. African-American soldiers made an important contribution to the war effort, without which slavery would not have been ended, but this movie was about political maneuvers, not the battlefield, or the underground railroad or slave revolts, so it was hard to show Black characters being proactive. Yes there were Black abolitionists, e.g. Frederick Douglass, but a realistic depiction of the politics of the 13th Amendment in Washington would inevitably be white-dominated.

Even so Spielberg and Kushner were criticized for their troubles re not having a bigger role for African-American characters:
http://www.nytimes.com/2012/11/13/opinion/in-spielbergs-lincoln-passive-black-characters.html