LONDON — When Queen Elizabeth II welcomes President Trump and other world leaders to Buckingham Palace on Tuesday to mark the 70th anniversary of NATO, it will be another testament to her extraordinary longevity: She acceded to the throne only three years after the alliance was formed.

Yet it will also showcase the House of Windsor at a wistful turning point, with the 93-year-old queen fading into history as her 71-year-old son and heir, Prince Charles, moves aggressively to assert his control, most conspicuously in trying to mop up the recent scandal that engulfed his younger brother Prince Andrew.

The fierce backlash over Prince Andrew’s television interview about his friendship with the disgraced financier and convicted pedophile Jeffrey Epstein has thrown a harsh spotlight on the queen’s management of the Firm, as insiders often refer to the royal family. More significantly, it has also dramatized how Prince Charles has effectively assumed the role of monarch-in-waiting.

In the aftermath of Prince Andrew’s disastrous BBC interview, in which he showed no empathy for the teenage victims of Mr. Epstein and offered dubious defenses of his own conduct, Prince Charles called his mother from New Zealand to press her to strip his brother of his public duties. The Prince of Wales was said to be worried that the scandal had spiraled so rapidly that it was threatening to eclipse this month’s general election in Britain, The Times of London reported.