Is interpretation of known historical facts protectable under copyright law?

Copyright protects the expression of ideas, not the ideas in themselves or historical facts per se. As pertains to the latter, what copyright will protect is prior accounts or interpretation of these historical facts fixed in any tangible form, whether in words, songs or other expressions of art. As long as you do not include these prior works belonging to other people in your own work without the permission of the owner first sought and received, I don't think you should have any problem pertaining to copyright. Just about any material on copyright or copyright law will shed more light on this.
I also feel the need to point out that proper acknowledgement of the source or ownership of the information (where decipherable) is very essential. Depending on the historical fact you are writing on, that knowledge or information may be unique to a particular group of people forming a part of their history, therefore a full, proper acknowledgement will be required in a case such as that.
I hope you (or anyone else) find(s) this helpful!