LONDON — It was a striking moment when Emmanuel Macron, newly elected president of France, torchbearer of a new politics, strode onto a courtyard of the Louvre to celebrate his victory: As the crowd cheered, waving the tricolor French flag, the choice of music was “Ode to Joy,” the anthem of the European Union. Some people even waved the bloc’s flag, with its circle of golden stars.

For the past year, and longer, the European Union has been politically radioactive, deemed untouchable by most mainstream candidates for national office in Europe. Yet Mr. Macron, 39, not only embraced the embattled bloc, he proclaimed membership of it to be a necessity for France’s future: needful of reform, certainly, but something to embrace rather than run from. And he defeated the most europhobic of opponents, the far-right nationalist Marine Le Pen.

Nathalie Tocci, director of the Institute for International Affairs in Rome, said Mr. Macron’s victory had helped the bloc avoid a cataclysm. “The alternative would have been the end of the European Union,” she said. “It means France is back in the picture.”

If France is again vital to European affairs, any euphoria is certain to be short-lived. First, Mr. Macron faces many domestic challenges in translating his centrist promises into policy and in assuaging those millions who voted for Ms. Le Pen, cast blank ballots or did not vote at all.