After seeing the film, I think some critiques are justified, while others are simply the product of critics being critics. Certainly, the film does have a recurring theme of how determined the government was to try and execute those who they saw as responsible for the assassination of Lincoln and the attempted assassinations of Secretary of State William Seward and Vice-President Andrew Johnson.
Perhaps the most important question raised was in regards to the validity of trying civilians in military tribunals. Not much of an explanation is given as to why this choice was made, other than, "the attorney general said it was ok, so...." The context for that decision was a bit more complicated than the movie lets on: not only were there still Confederate armies in the field at the time, but Stanton and others believed Booth and his conspirators were acting as agents of the Federal government. Indeed, history shows that the Confederacy paid Booth and some of his men for certain acts of espionage, but no funds were given for the assassination of Linoln and his cabinet members. Nevertheless, at the time, the link between the conspirators and the Confederacy was thought to be so strong that Jefferson Davis was personally charged with participating in the plot, along with Mary and the rest. So, while military tribunals may seem too harsh for 21st century audiences immersed in a debate over how to proceed with an international war on terror, their application at the time was in enormously different circumstances, few of which were mentioned by Redford. Using history to make modern political statements is always dangerous because it allows very little room for historical context, which often makes all the difference between truth and falsehood.
Perhaps the most interesting thing about the movie was how little it actually said about the guilt of Mary Surratt. Certainly, many suggestions were made regarding her innocence, but ultimately, her trial in the film came down to a matter of he said versus she said. However, according to the historical record, the case against her was a bit more convincing than it was portrayed as in the film. Kate Clifford Larson's book The Assassin's Accomplice makes a very strong case that Mary was indeed guilty and complicit in the murder of Abraham Lincoln. Not only was Booth a frequent guest at the Surratt boarding house, but he often spoke with Mary in private. On the day of the assassination, Mary transported materials Booth would need later that night. She was a staunch supporters of the Southern cause and the Confederacy and kept a photograph of Booth in her bedroom (the film makes some mention of this, noting that Anna Surratt was infatuated with Booth, as were many of the women associated with the Surratt boardinghouse). While Mary did not pull the trigger, she had numerous opportunities to observe Booth and his henchmen, understand what was going on, and support it by offering her boardinghouse as a meeting place. Thus, according to what I have read on the matter, Mary was indeed guilty. Including such conclusions in the film would detract from the sympathy built up for Mary, sympathy which plays along with the film's theme of an abusive government trashing civil liberties in a time of war.
On another matter, Mary's attorney, Frederick Aiken, was not quite the 19th century Johnny Cochran that was portrayed in the film. According to Larson's book, Aiken was a novice who stumbled into numerous lines of questioning that further damned his client, albeit not always according to his own faults. His job was no doubt made extremely difficult by legal procedures and his lack of access to prosecution witnesses and evidence, forcing him to ask questions for which he had no answers. Yet, according to the film, Aiken was a heroic Union soldier and a talented young attorney who fought to little avail to free the noble Mary Surratt.
Overall, I think the Conspirator is worth seeing, as long as one keeps in mind that Robert Redford is the director. Rarely does Hollywood put forward a 100% accurate and unbiased portrayal of historical events. With a topic of such intense debate among historians to this day, one would be hard pressed to make a film with an storyline about Mary Surratt and the Lincoln assassination on which historians would agree. On this count, a certain degree of leeway must be afforded to Redford. The film does have its merits. It was beautifully shot (from what I have read, it was made in Savannah), the main roles are well acted, and it communicates some of the traumatic emotions that pulsated throughout the nation at the time of Lincoln's death. I always find it fascinating to see such famous episodes (such as the Lincoln assassination) protrayed on the big screen. On these counts, The Conspirator is sure to satisfy the history buff and novice alike.
As for its primary theme and message, civil liberties in wartime is a subject which is little understood. Historian Mark Neely, who wrote the definitive book on Lincoln's actions regarding civil liberties during the war (which won the 1992 Pulitzer Prize), provides the best advice to keep in mind when trying to understand situations such as the trial of Mary Surratt. Understanding the conditions, context, and reasons for historical actions is much more important than using the past to make blanket statements about the present:
"If a situation were to arise again in the United States when writ of habeas corpus were suspended, government would probably be as ill prepared to define the legal situation as it was in 1861. The clearest lesson is that there is no clear lesson in the Civil War--no neat precedents, no ground rules, no map. War and its effect on civil liberties remain a frightening unknown." (Mark Neely, The Fate of Liberty: Abraham Lincoln and Civil Liberties, New York: Oxford University Press, 1991, 235.)