Former U.S. President Barack Obama delivers a speech in Berlin on May 25 as German Chancellor Angela Merkel listens | John MacDougall/AFP via Getty Images Obama in Berlin: ‘We can’t hide behind a wall’ Obama made the comment as President Donald Trump, in Brussels, was meeting with EU leaders.

Barack Obama brought his post-presidency campaign for continued globalism and openness to Berlin on Thursday, urging youth leaders and others not to give up and turn in on themselves.

"In this new world we live in, we can’t isolate ourselves. We can’t hide behind a wall," Obama said, sitting on stage next to German Chancellor Angela Merkel in front of the Brandenburg Gate at an event that’s part of "Kirchentag," a multi-day meeting sponsored by the Protestant church in Germany.

Obama made the comment as President Donald Trump, in Brussels for his first NATO leaders summit, was meeting with European Council leaders Donald Tusk and Jean-Claude Juncker.

Obama addressed a rapt crowd of Germans lining the streets from blocks away, some holding signs like, “Welcome Home,” “You’re Looking Great,” and “Du Bist Ein Berliner.”

Obama attended the event at Merkel’s invitation—there’s no foreign leader that he was closer with during his time in the White House, and she’s hoping for a boost from his still sky-high popularity in Germany in her own re-election campaign in the fall.

Obama warned against leaders who don’t question themselves.

"If I become so convinced that ‘I’m always right,’” Obama said, “the logical conclusion of that often ends up being great cruelty and great violence.”

Merkel's having to balance the line between the two presidents, one of whom she adores and the other of whom she does not: she had breakfast with Obama in Berlin before the event, and she's set to fly to Brussels later Thursday to have dinner with Trump.