G20 leaders gathered Sunday in South Africa hailed multilateralism — even as they struggled to adapt to a changing world order beset by go-it-alone U.S. policies, wars and deepening geopolitical rivalries.

The final day of their weekend summit — boycotted by the United States — kicked off with a searching discussion on how the G20 can survive in a fragmenting world.

“We are not experiencing a transition, but a rupture,” acknowledged Canadian Prime Minister Mark Carney to journalists just before the summit session.

“Too many countries are retreating into geopolitical blocs or the battlegrounds of protectionism,” he said, but added: “In every rupture resides the responsibility to build — nostalgia is not a strategy.”

Dozens of leaders from key economies around the world — including Europ

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