Pressed at a news conference earlier in the day about Russian interference in the American election, he said that ânobody really knowsâ if other countries might have been involved. He blamed President Barack Obama for not responding publicly after Mr. Obama learned about reports of possible election meddling last summer.
Mr. Trump â who is under pressure to confront Mr. Putin during their first face-to-face meeting in Hamburg, Germany, on Friday over his attempts to sway the election â delivered a mixed message on Russia, one tailored for his Polish audience, the other straight out of his Putin playbook.
The president made his sharpest criticism of Moscow since taking office, urging Russia to âcease its destabilizing activities in Ukraine and elsewhere and its support for hostile regimes, including Syria and Iran,â and asserting that it must âinstead join the community of responsible nations in our fight against common enemies and in defense of civilization itself.â
And Mr. Trump moved to reassure Poland and other allies fretful about Russiaâs aggression, making a full-throated endorsement of the collective defense principle that undergirds NATO, something he was unwilling to do during his first trip to Europe in May.
âThe United States has demonstrated not merely with words but with its actions that we stand firmly behind Article 5, the mutual defense commitment,â Mr. Trump said.
But he also said he was not entirely convinced that Russia was solely responsible for interference in the 2016 election, breaking with American intelligence agencies, which have agreed that the efforts emanated from Moscow and were directed by Mr. Putin.
âI think it was Russia, and it could have been other people in other countries,â Mr. Trump said when asked for a yes-or-no answer to the question about Russian meddling. âNobody really knows for sure.â
To back up his message about uncertainty, he recalled the intelligence failures that preceded President George W. Bushâs decision to invade Iraq in 2003. âEverybody was 100 percent sure that Iraq had weapons of mass destruction,â Mr. Trump said. âThey were wrong and it led to a mess.â
âWeâll see what happens â I donât like to talk about what we have planned â but I have some pretty severe things that weâre thinking about,â Mr. Trump said at the news conference, standing next to his Polish counterpart, Andrzej Duda. âThey are behaving in a very, very serious manner, and something will have to be done about it.â
The trip to Warsaw gave Mr. Trump an opportunity to showcase his willingness to defend Poland against aggression in the face of threats from Russia, and to helping American workers. He praised Mr. Duda for moving forward with the purchase of the Patriot missile defense system from the United States, which he called âthe best anywhere in the world.â
Mr. Trump emerged from a Marriott hotel in Warsaw on Thursday a little after 9:15 a.m., and his sprawling motorcade rode along the Vistula River to a back entrance to the presidential palace. He was greeted by Mr. Duda, and he disappeared for closed-door meetings after a session with photographers, emerging only for the news conference.
Unlike what is expected in Hamburg, where leaders of the Group of 20 economies will meet on Friday, there were no major protests in Warsaw, although there were signs of dissent.
Michael Schudrich, Polandâs chief rabbi, and other Jewish leaders criticized Mr. Trumpâs decision not to visit a monument to the 1943 ghetto uprising.
Every American president and vice president who has visited Warsaw since the fall of communism in 1989 has visited the monument. âWe deeply regret that President Donald Trump, though speaking in public barely a mile away from the monument, chose to break with that laudable tradition, alongside so many other ones,â the statement read. âWe trust that this slight does not reflect the attitudes and feelings of the American people.â
Hours after the Jewish leaders issued their rebuke, the White House sent word that Ivanka Trump, the presidentâs daughter and senior adviser, who is an observant Jew, had visited the ghetto site and laid a wreath at the monument there, visiting the Polin Museum of the History of Polish Jews.
In a statement distributed to reporters, Ms. Trump said her visit was âa deeply moving experience.â
âIt was a privilege to pay my respects and remember, with gratitude, those who tenaciously fought against all odds,â Ms. Trump said in a statement that did not mention Jews or the Holocaust. âThe monument, erected on the rubble of the Warsaw Ghetto, symbolizes the fight for freedom. I am profoundly grateful for those who fought and all those who continue to fight today.â
Mr. Trumpâs speech in Krasinski Square, which memorializes the Polish peopleâs resistance to tyranny, was well received, as was his message likening the fight against the Islamic State to Polandâs resistance of German invasion and occupation during World War II.
âWe must stand united against these shared enemies to strip them of their territory, their funding, their networks and any form of ideological support,â Mr. Trump said. âWhile we will always welcome new citizens who share our values and love our people, our borders will always be closed to terrorism and extremism.â
The pro-Duda crowd at Krasinski Square, where many waved American and Polish flags, serenaded reporters from both countries with periodic chants of âfake news.â
That came about an hour after Mr. Trump tag-teamed with Mr. Duda in a transnational denunciation of journalists who write negative stories about them.
What made Mr. Trumpâs sermon against the mainstream news media different this time was that Mr. Dudaâs center-right party, Law and Justice, proposed restricting the news mediaâs access to Parliament last year. The government backed down after street protests.
âThey have been fake news for a long time,â Mr. Trump said of CNN when asked about the tweet, adding that the network had been covering him in âa dishonest way.â
âWe donât want fake news,â he continued, as Mr. Duda nodded vigorously in agreement.
Mr. Duda, responding to an American reporterâs question about his own actions toward the news media, blamed Polish journalists for intentionally distorting his record and for failing to include his positions in articles critical of his government.
After chastising CNN â a go-to move on both sides of the Atlantic â Mr. Trump went after NBC, his former employer. âNBC is nearly as bad, despite the fact that I made them a lot of money on âThe Apprentice,â â he said.
Krasinski Square is considerably smaller than Zamkowy Square, outside the Royal Palace, where Mr. Obama spoke in 2014.
Worried that crowds would not show up on Thursday â Mr. Trump is less popular in Polandâs liberal capital than in the conservative countryside â the authorities chose a smaller, though still symbolically rich, site.