Royal-watchers have a new reason to celebrate. The British monarchy is set to stage its next royal wedding — starring an American.

Prince Harry, who is fifth in line to the throne, and Meghan Markle, an actress from Los Angeles, went public with their engagement on Monday and are scheduled to wed next spring.

Like previous royal nuptials, it is expected to be an eye-catching spectacle of pageantry, observing centuries of tradition carried through generations of knot-tying. But the coming union has also inspired feverish commentary that highlights the modernizing of the British monarchy.

The couple’s relationship has been scrutinized since the news that they were dating emerged over a year ago. Last year, Prince Harry lashed out at the racial undertones of British tabloid coverage of Ms. Markle, who is biracial and divorced, as not being British enough for him.