What makes a nation? For many countries, it is the possession of an army that ultimately keeps them in being, and in one piece. The old jibe that a language is a dialect with an army – and that a dialect is a language without an army – has a lot of truth in it. You can have a pretty flag and a tuneful anthem but if, ultimately, you cannot defend yourself when attacked, you are not a real nation and must live as your stronger neighbours dictate, or vanish altogether. In sea-girt Britain, this has always been different. For...