Speaking to youth leaders in Berlin, Germany, former President Obama advocated for openness in the international community, saying that global citizens could not "hide behind a wall."
"In this new world we live in, we can’t isolate ourselves. We can’t hide behind a wall," said the former president, sitting on stage next to German Chancellor Angela Merkel at an event that was a part of "Kirchentag," a multi-day meeting sponsored by the Protestant church in Germany.
Obama's comments came as President Trump prepared to attend his first NATO leaders summit.
Germans lined the streets to see the former president, holding signs that read "Welcome Home," and "You're looking great."
Obama is very popular in Germany, and Merkel's invitation is timely considering her imminent re-election campaign.
Obama also warned against leaders who don't question themselves and reconsider their decisions.
"If I become so convinced that I’m always right, the logical conclusion of that often ends up being great cruelty and great violence,” said Obama.