Obama will huddle Monday in Amsterdam with other members of the G-7, seeking a strategy against what many see as the most threatening European land grab since World War II. He will have to navigate disagreements among the European nations over how far to go, and the price they are willing to pay, to sanction Russia for seizing the peninsula from Ukraine.

And he will have to reassure Russia’s nervous neighbors that the North Atlantic Treaty Organization is prepared to uphold its promises to defend them.

It’s a retro role for the president, as well as for Western Europe, which has been snapped back into its pivotal place in global geopolitics. And it’s not one that Obama — who has spent his presidency talking about shifting U.S. policy away from the Old World — is likely to have anticipated.

Some say he has not prepared for this challenge.

“It’s going to require an enormous amount of American leadership in Europe, which we have not seen for the last five years,” said Heather Conley, director of the Europe program at the Center for Strategic and International Studies, a Washington think tank. “We’ve got a lot of making up to do.”