U.S. president speaks to thousands of Poles during his second trip to Europe.

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.

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.

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.”

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