Part of the reason why Cooper's work might not be historically accurate is because his purpose is not to create a work of history. As an integral part of American Romanticism and Transcendentalism, Cooper is more concerned with the development of new components of American Identity. He is more concerned with discussing the individual's relationship with nature, exploration of American "ruggedness," and developing a new consciousness that is not steeped within the confines of industrialized and cosmopolitan society. Cooper's primary focus is within this realm, and the notion of historical retelling is not his singular motivation.
Yes, I agree with what has previously been said; however, I would perhaps not even go so far as to position Cooper as an author of the historical novel. He is more often regarded as a romantic writer or a writer of the adventure narrative. While it is true that the war he depicts took place, the text is not focused on the war itself which makes it difficult to read it as a historical novel. Rather, it is a story that centers on race ( Cooper writes while the injustices of Native American dispossession being played out in the background) and adventure. It has the typical romantic hero as well that would establish himself as the archetype in American fiction.
In Harry E. Shaw's The Forms of Historical Fiction: Sir Walter Scott and His Successors, (Ithaca: Cornell University Press, 1983), he states that Scott "captured the immense power and spirit of historical Scotland and dramatized it so well that the people and places came to life."
The sub-genre of historical fiction uses real historical events or real people and dramatizes the life and times in which the author is writing. No one did it better than Walter Scott in his novel Ivanhoe.
In The Last of the Mohicans, Cooper used an immense amount of detail to depict the world as it was in 1775, though in real life, the Indians were Mohegans and not Mohicans, and many of the fictional characters did not exist. But it does not detract from the ways in which life existed back then and gives you some idea of not only what life was like, but also an idea of a real event surrounding the death of Jean McCrea in 1777 at the hands of Algonquin Indians.
In some aspects the novel is realistic and in others it isn't. It is common knowledge that different Indian tribes fought with one another and also served with different sides during the French and England. The wars were called the French Indian Wars. The English competed with France for the vast undeveloped areas America had to offer. The Wars were long and bloody and many people suffered both white people and Indian nations.
Fort William Henry was a British stronghold that was often being sought after. The British were able to divert and stop four French attacks. On August 9, 1757 the French took hold of the fort. The Indians were allowed to pillage and take whatever they wanted. There was not much left as the French had taken what they had wanted and only left behind wounded and dying British soldiers. The Indians killed them off.
The events listed above correlate to the story, but it is doubtful that the tale of the two Indians actually existed. In addition, they were moving women away from the fortress instead of to it.