WARSAW—In a bid to broaden the nationalist vision he has long embraced, President Donald Trump on Thursday described the West as locked in a struggle it could lose unless it can “summon the courage” to see it through.

Mr. Trump chose Poland as the backdrop for a defining foreign policy speech of his early presidency, calling the country’s perseverance in World War II and afterward a model for Western nations that face sinister threats of their own today.

“The story of Poland is the story of a people who have never lost hope, who have never been broken, and who have never, ever forgotten who they are,” Mr. Trump said at Krasinski Square, site of a memorial to a 1944 Polish uprising against the Nazis. His speech came a day before he was to meet for the first time with Russian President Vladimir Putin.

He exhorted the West to recognize the existential peril embodied by terrorists who have struck repeatedly at centers of Western arts and culture, including Paris, London and New York.

“We are confronted by another oppressive ideology—one that seeks to export terrorism and extremism all around the globe,” Mr. Trump said. “America and Europe have suffered one terror attack after another. We’re going to get it to stop.”

With Thursday’s address, the U.S. president sought to provide an intellectual grounding for some of the controversial policies he has pushed since taking office: the travel ban, building a border wall, and aggressive actions against illegal immigrants.

All these initiatives have faced setbacks. Courts have delayed and constrained Mr. Trump’s efforts to restrict travel from six Muslim-majority countries he says pose an elevated risk of terrorism. It isn’t clear whether he will win congressional support or funding for the wall along the U.S.-Mexican border, a linchpin of his effort to stop illegal migrants.

Detractors have said Mr. Trump’s moves reflect an anti-Muslim, nativist bias evident from the earliest days of his campaign. But in Mr. Trump’s telling, his steps are needed to fortify a Western culture at risk of being washed away.

“The fundamental question of our time is whether the West has the will to survive,” Mr. Trump said, amid chants of “Donald Trump! Donald Trump!”

“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? Do we have the desire and the courage to preserve our civilization in the face of those who would subvert and destroy it?” he asked.

A main architect of the speech was Stephen Miller, a senior adviser and part of a populist-nationalist wing at the White House led by strategist Steve Bannon, White House aides said.

At times Mr. Bannon’s clout has seemed in doubt. He has clashed with the president’s son-in-law, senior adviser Jared Kushner, and at times Mr. Trump has seemed to lose patience with him. But the Bannon-Miller faction doesn’t appear to be in retreat, and White House aides indicated they were pleased with Mr. Miller’s work. As Mr. Trump flew from Poland to Germany for a summit meeting, reporters on the plane could overhear aides congratulating Mr. Miller on the speech.

The president told The Wall Street Journal in the spring that Mr. Bannon was merely “a guy who works for me.”

In tone and substance, the speech departed from the typical pattern of Mr. Trump, who relishes the instant impact that Twitter provides in 140-character bursts. A senior adviser who briefed reporters on the speech shortly before its delivery said the aim was to portray Mr. Trump’s positions with more philosophical sweep.

Thursday’s address had a loftier ring than his address in Saudi Arabia in May, when Mr. Trump said America’s global role should be guided by what he called “principled realism.” That approach, as he described it, emphasizes transactions on economic and security agreements over other concerns, such as human-rights abuses.

“We will make decisions based on real-world outcomes—not inflexible ideology,” he said then in remarks before Muslim leaders.

The senior adviser said of Thursday’s address: “The core theme of this speech is a defense of Western civilization.”

The message isn’t necessarily an easy one for Mr. Trump to pull off. While he celebrated traditions of “free speech” and “free expression” in his speech, he has faced mounting criticism over his broadsides against news outlets reporting on election interference and a federal investigation into Trump associates’ possible collusion in attempts by Russia to damage Hillary Clinton’s presidential bid.

On his trip abroad, he kept up that criticism of U.S. news outlets. At a news conference in Warsaw, Mr. Trump was asked about his dust-up with the CNN after he recently tweeted a video portraying him wrestling a logo of the network to the ground. “What we want to see in the United States is honest, beautiful, free press,” he said. “We don’t want fake news.”

On Friday, Mr. Trump is scheduled to hold a bilateral meeting with Mr. Putin, a figure he praised during his presidential campaign, at a summit of leaders from the Group of 20 leading nations. U.S. intelligence agencies have concluded Russia meddled in the election with a goal to elect Mr. Trump.

It was unclear whether Mr. Trump would bring up the matter or caution Mr. Putin not to try interfering again. At his news conference on Thursday, Mr. Trump said “no one really knows for sure” who was behind the interference.

Far from guaranteeing Western civilizational norms, Mr. Trump could be coaxed into abandoning them if he isn’t careful in his dealings with Mr. Putin, said U.S. Sen. Chris Coons, a Delaware Democrat who serves on the Senate Foreign Relations Committee. “There’s a significant risk that Putin will play to Trump’s ego and will attempt to pressure him to abandon what are our core American traditions,” Mr. Coons said.

In Thursday’s speech, Mr. Trump criticized Moscow for its interference in Ukraine and its support for governments in Iran and Syria.

“We urge Russia to cease its destabilizing activities in Ukraine and elsewhere, and its support for hostile regimes—including Syria and Iran,” Mr. Trump said.

But some European officials wondered if Mr. Trump would carry those criticisms into Friday’s meeting.

“There’s no doubt that President Trump’s position regarding Russia is, on many occasions, different than what he presented today in Warsaw,” said European Council President Donald Tusk, a former center-right Polish prime minister. “I understand this, the audience one’s addressing often dictates the tone.”

German Chancellor Angela Merkel, who met Mr. Trump on Thursday in Hamburg, said the U.S. leader’s trip to Poland—which some Western European politicians had feared could deepen rifts on the continent—didn’t worry her “at all.”

“We have our agenda here, but there are different conceptions of globalization,” Ms. Merkel said, previewing the G-20 summit. “There don’t always have to be losers where there are winners.”