The biggest concern the White House staff has about the upcoming G20 summit isn’t what Europe might do, or what China might do, or even what North Korea might do in response to any finger-shaking by world leaders. The White House is more concerned about the actions of Donald Trump. Specifically, they don’t know if Trump will be able to contain himself when he meets his idol.

Even his top aides do not know precisely what Mr. Trump will decide to say or do when he meets President Vladimir V. Putin of Russia face-to-face this week on the sidelines of the Group of 20 economic summit gathering in Hamburg, Germany. And that is what most worries his advisers and officials across his administration as he embarks Wednesday on his second foreign trip, first to Warsaw and then to Hamburg.

It seems almost certain that Trump will avoid the elephant in the room—since he will enter the room riding that elephant—and there will be no discussion with Russia about their interference in the 2016 election, or the threat they pose to 2018 and beyond. Instead, Trump will probably talk about Syria (where Secretary of State Rex Tillerson recently said Russia gets to decide the future of the nation). And they will certainly talk about the sanctions that stand between Russia and the enormous payday Tillerson arranged before moving from Exxon Corp. to Trump Corp.

Mr. Putin has signaled that he will press Mr. Trump to lift sanctions imposed on Russia for its annexation of Crimea, its interference in Ukraine and its election meddling, and to hand over Russian diplomatic compounds on Long Island and in Maryland that the United States seized last year.

Meanwhile, the focus of other G20 leaders seems to be on working out the shape of a world where American leadership has collapsed.

Merkel initially said Europe can no longer rely on the United States during a campaign event shortly after the Group of 7 summit in Italy in late May. She reiterated that position in an interview with the weekly Die Zeit that was published two days before she hosts U.S. President Donald Trump and other world leaders.

While White House officials may be worried about Trump, Trump is mostly worried that someone will say something mean to him about his earth-threatening, poor decisions.

Mr. Trump himself is not troubled by the meeting [with Putin]. He has told aides he is more annoyed by the prospect of being scolded by the German chancellor, Angela Merkel, and other leaders for pulling out of the Paris climate accords and for his hard line on immigration.

As for Putin, Trump’s staff worries that their blow-hard bully will swoon for an experienced, freedom-crushing autocrat who doesn’t stop at punching images of news media.

The biggest concern, people who have spoken recently with members of his team said, is that Mr. Trump, in trying to forge a rapport, appears to be unwittingly siding with Mr. Putin. Like Mr. Trump, Mr. Putin has expressed disdain for the news media, and he asserted in a recent interview that secretive elements within the United States government were working against the president’s agenda. Two people close to Mr. Trump said they expected the men to bond over their disdain for “fake news.”

With the United States pulling away from big decisions and dropping out of participation in the Paris agreement, other nations are filling the void.

The U.S. traditionally takes point in the search for common approaches to the big global issues of the day at G-20 summits. Not this time.

When world leaders meet in Hamburg on Friday, China and Germany will move in to usurp the U.S.’s role.

The two industrial powerhouses of Asia and Europe are being nudged into an informal alliance to pick up the leadership baton that the U.S. is accused of having dropped since President Donald Trump’s inauguration earlier this year, according to diplomats and officials from several Group of 20 members.

Even on the left, there are people who cheer Trump's "let Russia/China/Germany do it” isolationism as an end to American Exceptionalism. It's not. “Walking away from the table” is just another expression of exceptionalism—one that not only demonstrates an indifference to traditional partners, but one that violates the "that good men should look on and do nothing" rule

Leadership doesn’t mean going it alone. Good leadership means exactly the kind of compromise, team-building, and consensus generation that the right often disdains. Leadership doesn’t mean “my way or the highway.”

To put it in terms of a (clumsy and well-worn) sports metaphor, bullying the other players doesn’t help. Neither does taking the ball and going home. 

Isolationism isn’t an acceptance of blame: it’s an avoidance of responsibility. Partner that attitude with a still-growing military and an attitude that threats to the nation are also increasing, and it’s a formula for disaster—and for diminishment.

“China and Germany’s new closeness is something that happened because of the Trump episode,” said Diego Ramiro Guelar, ambassador to Beijing for G-20 member Argentina. “The two most important leaders in the world are President Xi and Chancellor Merkel at the moment.”