(CNN) As President Donald Trump makes his foreign debut in Brussels next month, his predecessor Barack Obama will be sharing a stage on the same day with a key European ally across the border in Germany.

Obama's presidential foundation said the recently departed commander-in-chief would join German Chancellor Angela Merkel in Berlin to mark the Protestant church's 500th anniversary, his first public appearance abroad since leaving office. Obama was invited to attend last year, while he was still in the White House, according to former administration officials.

Trump, meanwhile, plans to travel overseas for the first time as President for a summit of NATO leaders in Brussels, where the defense alliance is headquartered.

NATO Secretary General Jens Stoltenberg, who is visiting the White House on Wednesday, announced last month the summit was scheduled for May 25. The White House later confirmed Trump planned to attend the gathering.

The dueling appearances weren't intentional, according to German and US officials. But they do come as ties between Washington and Europe enter an uncertain phase, spurred by Trump's vague pronouncements of foreign policy and a contentious election season for European leaders.

During his appearance in Berlin, Obama plans to discuss the importance of participating in democracy with Merkel, who is up for election herself in the fall. The event is due to take place on a specially constructed platform at the Brandenburg Gate, a symbol for Democracy where US presidents -- including Obama -- have spoken before.

The event kicks off a summer marking the quincentennial of the Protestant Reformation.

German officials declined to say Wednesday whether Merkel also expects to make it to Brussels for the meeting of NATO leaders, which includes her. Germany's commitment to the defense alliance has become a major sticking point between Merkel and Trump, who met last month at the White House.

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In that Oval Office sit-down, Trump repeatedly pressed Merkel on Germany's defense spending, which does not yet meet NATO's goal of reaching 2% of gross domestic product. Merkel has insisted she's committed to raising military spending, but Trump's aggressive stance rankled some German officials, who wondered whether Trump's single-mindedness on the topic might forestall cooperation on other areas.

The issue of defense spending is likely to arise again in Brussels, where Trump will make his first overseas stop as president. Trump has lagged behind previous US leaders in foreign travel, who all made stops outside the US within their first three months in office. After his stop in Belgium, he's set to travel to Sicily for Group of 7 talks, which Merkel is expected to attend.

In appearing with Obama in Germany, Merkel hopes to capitalize on some of the lingering nostalgia that remains for the Democratic leader there. Obama said during his final visit to Berlin in November that "if I were German and I had a vote I'd support her" in her reelection bid.

Trump, who is deeply unpopular in Germany, is set to visit the country in July for a Group of 20 meeting in Hamburg, two months before Germany's federal elections.

Obama has largely remained quiet since departing office in January. He's made no public appearances, instead spending long stretches at various tropical escapes working on his forthcoming memoir.

He is due to accept an award at the John F. Kennedy Library in Boston in early May, his first scheduled public appearance since Trump was inaugurated in January.

Ties between Trump and Obama have unraveled in the beginning stretch of the new administration after Trump accused his predecessor of wiretapping the phones at Trump Tower, an unfounded allegation that Obama's representatives have denied.

While aides to both men spoke in the aftermath of Trump's claims, the two presidents have not themselves discussed the matter.

Trump told the New York Post in an interview Tuesday that he's had no recent contact with Obama, and maintained his claim that he was unlawfully wiretapped by the previous administration.

"I'm very disappointed that I was surveilled and so was my campaign," Trump told the newspaper. "That's not supposed to be happening, but I've been proven right."