"A nation is a lot of people who think they are a nation" was my old history teacher's definition. Not, prima facie, very helpful, but it emphasises the 'tribal' nature of it, of all belonging and acting and thinking accordingly. He was interested how, in history, nations suddenly emerged and equally suddenly disappeared, and nationalism with them. Often a 'nation' disappeared simply because the members of it became part of a larger body of people who all saw themselves as a nation, a bigger tribe. Mercian nationalism seems to have died out ! But if any group remembers , or comes to think of itself as, being Mercian, and sees the rest of its present nation as against it, you never know.