It's kind of comical how Krugman sees history  the way that some cherry pick the Bible to defend their idiocy.  For anyone even remotely familiar with Roman history, they'd know that the fall of Rome even the republican era was a long a complicated process brought on by a multitude of factors and obviously it's easy to just compare electing Trump to the fall of Rome and pat yourself on the back for bringing some ancient historical perspective to your plebeian readers.  He probably expects us to go forth and quote his historically cerebral comparisons to friends and family so that they can be impressed.  Some will, the ones who've cracked a history book will smirk.