Timothy Garton Ash, a contributing editor to Opinion, is professor of European studies at Oxford University, a senior fellow at the Hoover Institution and the author, most recently, of "Facts are Subversive: Political Writing from a Decade Without a Name."

If things continue as they are and Britain's Prince Charles succeeds his mother to reign as king until his death at a ripe old age, then sometime around 2040 the young couple getting married in Westminster Abbey on Friday will be King William V and Queen Catherine. By sheer accident of birth, William will then be the head of state of whatever is left of today's United Kingdom. Would that be all right? My answer is: In theory, no; in practice, probably yes.

If William and Kate behave themselves, unlike some of the gamier members of Britain's royal family, and contribute to the development of a modernized, slimmed-down constitutional monarchy, this can actually be better than the likely alternatives. As I look across Europe, I don't think countries such as Sweden, Holland, Denmark and Spain, all of which have monarchs, are worse off than those that have party politicians directly or indirectly elected to be president. Or would you rather have Buckingham Palace occupied by a President Tony Blair?

With one brief interlude in the 17th century, when English revolutionaries experimented with decapitating one of them, there have been kings and queens for more than 1,000 years. That is an amazing thing. It is the stuff of poetry. Imagine Shakespeare purged of all references to kingship. Before you abandon 1,000 years of poetry, you should be very certain that you will fare better in prose.

As we see again with the world media invasion of London for this week's royal wedding, this history, legend and mystique is also a significant contributor to Britain's soft power (the power to attract) and its earnings from tourism. I don't think anyone goes to Berlin to watch them changing guard at Bellevue Palace, or to catch a glimpse of President and Frau Wulff, and the little Wulffs. "President who?" is what most of the world would ask, if reference were made to the current head of state of Europe's most powerful country....