The museum covers antiquity to 1974, with special focus on the Balkan Wars, and World War 2. It does not cover the Civil War. It has fewer artifacts than the National Historical Museum, but more pictures and art.
For those of you who might not get a chance to see it, or who would like to check out pictures, here is a massive amount of pictures. I will not put captions beyond some points.
Greek fighter jets parked in the museum's courtyard