LONDON—Millions of people are expected to watch on Saturday as the archbishop of Canterbury places a jewel-encrusted gold crown on King Charles III ’s head to the sound of trumpets and gun salutes in a coronation ceremony that dates back 1,000 years. The event in Westminster Abbey is one Britons haven’t witnessed for 70 years. It is the only coronation ceremony left in Europe, a throwback to a bygone age when all-powerful monarchs dictated the fates of nations. In the U.K., that regal power is now largely ceremonial, but the pomp and history remain.

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