PARIS — President Emmanuel Macron of France seemed to be everywhere at once during the Group of 7 summit. For the space of a weekend, at least, the West appeared to have one person running the show, and it was not the American president.

One day, Mr. Macron was wooing President Trump over a long, private lunch. The next he was flying in the Iranian foreign minister for unannounced talks. He seized the role as chief defender of the global climate, telling Brazilians to get themselves a new president. He prompted a surprise diplomatic opening on Iran from Mr. Trump, even if both initiatives hit early headwinds on Tuesday.

Mr. Macron missed no opportunity to wring every advantage from his role as host of the summit in the southern resort city of Biarritz. It gave him the perfect stage to pursue his ambition, both grandiose and self-serving, to position France, and himself, as candidates to fill the vacancy left by Mr. Trump’s retreat from traditional Western values.

With Mr. Trump deepening American isolation on major global issues, and Chancellor Angela Merkel of Germany on a glide path out of power, Mr. Macron has become the leading champion of European unity and multilateralism.