HAMBURG—Through marathon meetings and dinners at the Group of 20 summit, various world leaders sought to coax America’s new president to accept core tenets of the internationalist order they embrace, including commitment to free trade and tough environmental regulation.

He barely budged. In his second foreign trip, President Donald Trump largely held firm to the nationalist principles that were central to his campaign identity, suggesting that the America First doctrine that underpins his foreign policy is more durable than some of his European counterparts had hoped.

German Chancellor Angela Merkel, speaking Saturday at the close of the summit, said the ideological gulf has proved tough to bridge. Talking about the back-and-forth with the Trump administration over the summit’s joint statement, the German chancellor said: “The negotiations on the climate issue reflected dissent—everyone against the United States of America. And that the negotiations on the trade issue were especially tough was also a result of the United States taking certain positions.”

Still bristling over Mr. Trump’s decision to withdraw from the Paris climate accord, one French official said that an aim in drafting the joint statement was to ensure that the U.S. “is clearly identified as being alone.”

If anything, Mr. Trump’s two forays overseas have shown that some leaders are bending toward his positions, not the reverse.

The statement, for example, carried language about America helping other countries use “fossil fuels more cleanly and efficiently.” A European Union official conceded the reference to “these kind of energy sources is not something we like.”

But the White House liked it. Trump officials cast the wording as a victory for a president who still sees a place for oil and gas drilling.

“We got language in there that is consistent with the speech the president gave when he announced the decision to withdraw from the Paris accord,” one senior White House official said Saturday.

So it went with trade.

Mr. Trump has long complained that past trade deals favored other nations at the expense of the U.S., resulting in large trade deficits. The final statement bore a distinctly Trumpian ring, stressing the importance of “reciprocal and mutually advantageous trade.”

The White House was happy with it. When the subject of trade came up a few months ago at a meeting of finance officials from the 20 leading countries, “it was kind of 19-1—me being the one,” Treasury Secretary Steven Mnuchin recalled.

Far from showing that his views have shifted, Mr. Trump used the three-day trip to deliver the clearest vision he’s offered to date of his world view: a commitment to preserving western civilization. He held up the West as the standard all should reach.

The location he chose for that message was Poland, home to a socially conservative government that has at times antagonized European partners.

“The fundamental question of our time is whether the West has the will to survive,” Mr. Trump said. “Do we have the confidence in our values to defend them at any cost? Do we have enough respect for our citizens to protect our borders?”

As he did in Belgium in his first foreign trip in May, Mr. Trump called on NATO countries to pay their share of maintaining the alliance.

Thousands of Poles cheered him: “Donald Trump! Donald Trump!”

It made an impression. “Poland was so terrific to me,” he said at an event Saturday showcasing a women’s entrepreneurial fund that was his daughter Ivanka’s brainchild.

Mr. Trump did give a long-sought endorsement of NATO mutual defense measures in his Poland speech, but he also showed in his meeting with Vladimir Putin that he is intent on finding ways to work with Russia, as he has vowed to do since early in his campaign.

If Europe is wary of Mr. Trump, his core voters are likely to be reassured, analysts said. The Donald Trump speaking at Warsaw’s Krasinski Square was the one they remember from red-state rallies.

“He wants to show at least his domestic base that he’s true to all of the principles that he enunciated during the election campaign,” said Angela Stent, a government professor at Georgetown University.

Still, European leaders aren’t giving up on the idea that they can bring Mr. Trump around to their views.

One diplomat at the summit said Mr. Trump is something of a “work in progress” and that officials need to exercise patience in dealing with him.

As president, Mr. Trump is getting more of a firsthand look at how his internationalist counterparts actually work.

The women’s fund championed by his daughter might be one example. It is run by the World Bank, a symbol of the sort of multilateral institution Mr. Trump has at times scorned. Different countries are ponying up millions of dollars to get the fund up and running—to Ivanka Trump’s delight.

Mr. Trump heaped praise on World Bank President Jim Yong Kim at an event formally rolling out the fund on Saturday.

He also showed he’s willing to challenge Mr. Putin, who has been a pariah among many European leaders partly because of Russia’s 2014 intervention in Ukraine and support for Syrian leader Bashar al-Assad.

In their two-hour meeting Friday, Mr. Trump told Mr. Putin that Americans aren’t happy about Russian interference in the 2016 campaign. “I strongly pressed President Putin twice about Russian meddling in our election. He vehemently denied it. I’ve already given my opinion…..,” Mr. Trump tweeted Sunday morning. “We negotiated a ceasefire in parts of Syria which will save lives. Now it is time to move forward in working constructively with Russia!”

Earlier in the week, Mr. Trump equivocated on whether Russia was solely responsible for election meddling. Other countries could have done it, too, he said, though he didn’t specify which ones.

In an account Mr. Putin gave Saturday, Mr. Trump pressed him on the issue repeatedly. “He allocated a great deal of attention to this,” the Russian president said.

Mr. Trump’s third foreign trip comes later this week, when he travels to Paris to mark Bastille Day.

That visit could also test Mr. Trump’s principles, as he meets French officials wary of him moving in a protectionist direction.

“We have sent a message to Americans that if there are unilateral measures from the U.S. we will react and react very quickly,” one French official said.