Now, the stakes may be even higher.
Merkel, host of the G-20 summit, has been open about her disappointment in Trump's decision and told the German parliament "we can not expect easy talks in Hamburg" on climate issues.
While Washington insiders await the first face-to-face meeting between President Donald Trump and Vladimir Putin, the first stop of the president's European trip may carry more significance.
In that quest, the embattled United States president will get a sizeable assist from Poland's ruling conservative party, which said it is laying on free buses to ferry in admirers.
"The big question is whether he will fix some of the damage from the stops in Europe in May, or will they be compounded?" said Jeff Rathke, a senior fellow at the Center for Strategic and International Studies in Washington.
Now reports out of Poland suggest that the Polish government has figured out a way to make Trump feel comfortable in their country - by busing in people to cheer him.
Trump's remarks are expected to focus on the Polish-US alliance and his vision for transatlantic relations and will take place on a square that memorializes the city's valiant uprising against Nazi occupation.
"It's going to be huge - absolutely huge", said pro-government Polish politician Dominik Tarczynski, who is organizing a busload of Trump supporters.
Jarosław Kaczyński, head of the ruling party Law and Justice (PiS), gave his views on immigration at a party convention in Przysucha, 100 km south of Warsaw.
His first foreign trip was to Saudi Arabia and a mooted state visit to the United Kingdom appears to have been put on the backburner. And Poland is reportedly rolling out the AstroTurf for his visit, aiming to inflate Trump's popularity as he drops in for a brief visit. Organizers expect large-scale protests to accompany the summit. The Polish government will certainly hope for such a message, to help legitimise its anti-migrant and increasingly anti-EU stance, particularly in the eyes of its domestic audience.
Trump is already responding to Warsaw's fawning. Trump's administration has castigated her country's trade surplus, to be met largely by German scoffing.
On the campaign trail and now in the White House he frequently declares his events to be the "biggest ever" in that venue.
In May, leaders chafed at the lecture Trump delivered on defense spending during talks at North Atlantic Treaty Organisation headquarters in Brussels.
"We do not force anyone to come, we do not pay people for them to come. The same happens in the United States or in France" Bielan told AFP.
Trump is scheduled to meet with Chinese President Xi Jinping at the G20 summit in Germany this week.
Leaders in Europe hope Trump will offer assurances that he'll hold Putin to account for the long list of grievances they hold against the Russian leader -- including provocative cyber activity. "And they use it to blackmail us, to blackmail Ukraine", Warsaw University's Lewicki said.
USA presidents are nearly guaranteed a friendly reception in Poland, and this aspect of the trip will appeal to President Trump and make for good pictures at home.
"He'd like the United States and the entire West to develop a more constructive relationship with Russian Federation", said the White House national security adviser, H.R. McMaster. "But he's also made clear that we will do what is necessary to confront Russia's destabilizing behavior".
Trump will also walk a fine line when he meets Friday with Putin.