Now, the stakes may be even higher.
Returning to the continent late Wednesday, Trump and those same European leaders will give it another go, with expectations of fostering a newly warm relationship far lower. Instead, both sides expect further clashes between the US President and leaders like German Chancellor Angela Merkel, who is convening this year's Group of Twenty meetings in her hometown, Hamburg.
The northern port city in Germany is also the setting for Trump's first in-person meeting Friday with Russian President Vladimir Putin, a closely watched session that has taken on greater significance given the swirl of controversy about Russia's cyber meddling in last year's election.
Hamburg is expected to play host to confrontations over a broad slate of issues where Trump and his European allies have disagreed, mainly because Merkel has ensured those sticking points -- including trade, climate, and migration -- feature prominently on the G20 agenda. Putin, and the US relationship with Russia and future of NATO, also looms large.
"The big question is whether he will repair 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.
Before Trump arrives in Germany he's making a 15-hours-long stop in Poland, a country where he's likely to receive a positive welcome from a conservative government and a largely pro-American population.
Air Force One touched down at the Warsaw Chopin Airport just after 10 p.m. Wednesday.
On Thursday, he's due to address a regional summit of Eastern European leaders and deliver an outdoor speech in Krasinski Square, the site of a monument to Poles who fought to liberate Warsaw from German occupation in World War II. White House officials said the speech would spell out, in broad terms, Trump's vision for transatlantic ties.
Poland's right-wing government has caused anxiety in Brussels, where EU leaders fear proposed constitutional reforms and a crackdown on opposition media could amount to democratic backsliding. But those concerns didn't prevent Trump from scheduling a stop in the country, where he's expected to praise Poland's success in meeting its NATO defense spending pledge and tout new exports of US liquid natural gas in a bid to wean the country from Russian-produced energy.
And while Poles, like most of the world, continue to regard Trump with some skepticism -- mainly due to his unclear intentions toward Russia, with which they share a border -- the country's population is largely favorable to the United States on the whole, meaning he's expecting to be met with cheering crowds during his brief stay.
The visit to a friendly capital is similar to Trump's decision in May to make Saudi Arabia his first stop abroad, where he was given an elaborate royal welcome before traveling onward to more fraught destinations.
Cold welcome for Trump in Germany
But the plaudits are unlikely to follow Trump from Poland to the G20, which brings together the leaders of the world's largest economies. Organizers expect large-scale protests to accompany the summit. But even closeted inside the confines of the Hamburg Messe conference hall, Trump is unlikely to find much agreement on his agenda.
Merkel has designed her summit to highlight areas where she's already rowed with Trump, including on trade. Trump's administration has castigated her country's trade surplus, to be met largely by German scoffing. Last week, White House officials were angered when a video feed of US Commerce Secretary Wilbur Ross speaking to a Berlin economic conference about the trade dispute cut off mid-speech.
Trump tweeted about trade deals Wednesday morning.
"The United States made some of the worst Trade Deals in world history," Trump wrote before leaving for his trip
. "Why should we continue these deals with countries that do not help us?"
In May, leaders chafed at the lecture Trump delivered on defense spending during talks at NATO headquarters in Brussels. Later that week, at the G7 in Italy, Trump felt outnumbered, according to people who traveled with him, and picked on when the other leaders insisted he maintain the US commitment to the Paris climate accord.
After his return, Trump announced he was withdrawing from the pact, casting the 2015 agreement as a humiliation that led other countries to laugh at the United States.
Merkel, meanwhile, vented during now-infamous remarks at a campaign event held inside a Bavarian beer hall that Germany and Europe could no longer rely on the United States as a steady partner.
Merkel -- who is running for her fourth term as Chancellor -- hasn't tempered her tone since then. Previewing the G20 summit last week in remarks to the German parliament in Berlin, Merkel acknowledged the talks would be "difficult" and seemed to castigate the isolationist worldview that Trump has periodically espoused.
"Anyone who thinks the world's problems can be solved with isolationism and protectionism is simply delusional," she said.
She's included an entire session on climate at the G20 she is hosting, and has invited leaders from China and India — two of the world's leading carbon emitters, both of whom say they remain committed to the Paris accord — for separate meetings in Germany over the last month.
The goal, according to European diplomats, is to keep Trump isolated on the climate issue in the hopes he will come in line with the rest of the world's leaders. If he doesn't, Merkel and other leaders hope to convince countries like China to take on a new leadership role in that area.
Meeting Putin for the first time
During his time in Hamburg, Trump will meet separately with China's President Xi Jinping, as well as the leaders of eight other countries — an ambitious agenda whose highlight is Trump's meeting with Putin on Friday afternoon, their first chance to size each other up since Trump was elected.
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.
Trump is also under immense political pressure back home to raise the election meddling issue during his session with Putin, the logistics of which were still being sorted out by the White House and the Kremlin midweek. But multiple senior administration officials and people close to the White House said they didn't expect Trump to raise the issue, at least in a significant way, during his talks with Putin. Instead, they said the civil war in Syria and the crisis in Ukraine would top the agenda.
That could provide further grist for tensions between Trump and other world leaders, who fear the President's past praise of Putin and professed desire to work with the Russian leader could mean he won't take a hard line on territorial disputes or cyberattacks.
"As the President has made clear, he'd like the United States and the entire West to develop a more constructive relationship with Russia," said H.R. McMaster, Trump's national security adviser, in a briefing last week. "But he's also made clear that we will do what is necessary to confront Russia's destabilizing behavior."
McMaster, who said there was no set agenda for the meeting, suggested Trump was open to taking steps that would allow for a better relationship between Washington and Moscow. But that desire to find areas of agreement has sparked concern among national security experts, even within Trump's team, who worry Trump may be unprepared for the calculating and hyper-prepared Putin.
"If Vladimir Putin comes in with a big proposal, President Trump's alarm bell better be going off because it may sound really good, but there well may be some traps in there," said Steven Pifer, a senior fellow at the Brookings Institution and a former US ambassador to Ukraine. "It's not clear that those will be noticeable on the first meeting."