Trump, in his most forceful terms to date, reaffirmed the United States support for the North Atlantic Treaty Organization.
In a nod to Poland's long-held fears of Russia to the east and Germany to the west, Trump spoke about the need for an independent Poland to be able to rely allies like the United States to deter Russian aggression and of the right of Poland to control its own borders.
Trump, speaking behind thick panes of bulletproof glass, stood firmly behind NATO and Article 5, the alliance's commitment to mutual defense, yet chided other European countries for not spending more money.
"To those who would criticize our tough stance, I would point out that the United States has demonstrated not merely with words but with its actions that we stand firmly behind Article 5, the mutual defense commitment," Trump said. "Words are easy but actions are what matter and for its own protection, and you know this, everybody has to know this, Europe must do more."
With controversial words that will reverberate around Europe, Trump cast Poland as the "soul of Europe," aligning the United States with the Eastern European nation as it stands up to Russia and looks to buck aspects of the European Union.
"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," Trump said to cheers.
"Your nation," Trump said, "is great because your spirit is great and your spirit is strong. For two centuries Poland suffered constant and brutal attack but while Poland could be invaded and occupied and its borders even erased from the map, it could never be erased from history or from your hearts."
Trump said his message to Poland was simple: "America loves Poland and America loves the Polish people."
Thousands of Poles crowded among the Baroque buildings to hear Trump in his first outdoor address on foreign soil, a major presidential moment that past commanders in chief have used to explain or promote their policy agendas.
Trump -- who to took the stage as the audience chanted "Donald Trump, Donald Trump" -- was greeted with an array of pro-American signs, including flags from his 2016 campaign.
Melania Trump, Trump's Slovenian-born wife, drew loud applause introducing the President, her Slavic-accented greeting -- "Hello Poland!" -- prompting a round of approving cheers.
As spectators -- some bused into the Polish capital from far-flung provinces by civic committees -- streamed into the square, government officials handed out pairs of American and Polish flags. The bronze columns of the Warsaw Uprising Monument were decorated with red-white-and-blue banners as military police patrolled the crowd from atop nearby buildings.
The location was significant to those Trump aides who planned Trump's speech: Krasinski Square is dominated by a large memorial that remembers the Poles who fought, but failed, to free Warsaw from Nazi control in 1944.
Calling Trump's speech a "defense of Western civilization," a senior Trump administration official said "the setting of Warsaw uprising is poignant because it demonstrates vividly just how much has been sacrificed to preserve our civilization and society."
Warsaw was razed after the uprising and thousands of Polish resistance fighters were killed or taken prisoner. The city, especially the Old Town area, is now dotted with monuments and remembrances to the resistance fighters.
Trump heralded the "courage and strength deep in the Polish character that no one could destroy."
"Through it all," Trump said, "you never lost that spirit. ... Poland could not be broken."
Speaks on immigration and energy
Looking beyond Polish history, Trump pledged energy cooperation with Poland in a not-so-subtle knock against Russia's use of energy as a coercive power.
"We are committed to securing your access to alternative sources of energy so Poland and its neighbors are never again held hostage to a single supplier of energy," Trump said.
And in a win for the Duda and his Law and Justice Party, Trump stressed the right of a country to restrict migration into its country. Duda has bucked European Union regulations on refugees and ran, in part, on an anti-immigrant platform since 2015.
"There are dire threats to our security and to our way of life. You see what is happening out there, they are threats. We will confront then, we will win, but they are threats," Trump said. "Our borders will always be closed to terrorism and extremism of any kind."
'I love the US culture'
Young Poles who traveled hours into Warsaw on Thursday said they hoped to hear Trump affirm the United States' commitment to countering Russia's aggressions in Europe. Thousands of US troops are currently deployed as part of a NATO mission to protect against Russian incursions.
"We are afraid of Russia," said Michael Andrzejak, a 31-year-old farmer from Bobolice, who rode on a bus three hours to view Trump's remarks.
He said he expected Trump to discuss the military efforts in Poland to counter Russia, but also address issues of US visas, which young Poles rely upon to find work.
Gabi Siwniuch, 16, said she hoped Trump would address ways the US could help young Poles' job prospects.
"I love the US culture," she said, standing in an orderly crowd that had been waiting more than two hours to hear Trump.
Siwniuch said Trump was well-known in Poland — and largely admired for his candid style.
More than 1,000 were seated and standing in or near the square, but the crowd stretched down several streets. Government officials in suits and members of the military in their uniforms had the best seats. But young people and others casually dressed in T-shirts made up the rest of crowd in a large set of bleachers and on nearby streets.
Trump's jovial visit to Poland will stand in stark contrast to his visit to Hamburg, Germany, where contentious meetings with leaders skeptical of his leadership await.