When Gerry Adams rose to announce at his funeral that Martin McGuinness was no terrorist but a ‘freedom fighter’, the historian Thucydides probably allowed himself a grim smile. He knew all about these sort of people.

In 427 BC, Corcyra (ancient Greek Kerkura, now Corfu) was in the grip of a ferocious civil war between oligarchic and democratic factions for control of the state.