I found 3 different pages on this grammar an vs. a (which are found throughout the comments), two agreed with you, one with me (that it’s variable, depending on the pronounciation of “historian”. I haven’t found anything in strunk and white, though…
It’s a minor disagreement over grammar, and I’m happy to be wrong… No need for an argument… I mean, this is an article about Marx, and it hasn’t come to (virtual blows) yet… I refuse to let it do so over grammar… 
BTW— I looked it up in the Chicago Manual of Style (15th edition), and it says use an with words with consonants that sound like a vowel. I don’t know if other style manuals say different things… I forget which other fields use Chicago other than history… Journalism, maybe? Any English nerds know what MLAs style manual says?
