Or, we could ask, is Brexit like Passchendaele or like The Somme?
I mean, of course, whether the noun Brexit should normally be used with a definite article ("Are you for or against the Brexit?") or without ("Are you for or against Brexit?").
We need to ignore all the constructions in which Brexit is a modifier of another noun: the Brexit vote, the Brexit campaigners, the Brexit turmoil, etc. But when Brexit is the head of a noun phrase, I've been assuming that it's a strong proper name that should be anarthrous, like Christmas or Passchendaele or Language Log.