Complex sentence: The film was based uniquely on the very real history around the atomic bomb and as such it becomes the first ever historical film to come close to an almost documentary accuracy that could not previously have been achieved due to the secrecy over the A-Bomb project.
Split into simpler sentences: The film was based uniquely on the very real history around the atomic bomb. It is perhaps the first historical film to come close to an almost documentary accuracy in its portrayal of these events, an accuracy that could not previously have been achieved, due to the secrecy over the A-Bomb project.