WARSAW — The Trump administration reaffirmed its commitment to the NATO alliance and its core mission of a united front against Russia on Wednesday, but with a caveat that American interests will always come first.

Vice President Pence, Secretary of State Mike Pompeo and acting defense secretary Patrick Shanahan were all in Europe expressing solidarity with the nearly 70-year-old alliance that President Trump has at times questioned as obsolete and described as an undue burden on the United States.

“The United States of America stands with Poland in the most successful mutual-defense alliance in the history of the world — an alliance that each of you serves to uphold and defend — the North Atlantic Treaty Organization,” Pence said as he met Polish President Andrzej Duda and addressed armed forces from Poland and the United States. “Under President Trump, the United States will always put the security and prosperity of America first. But as the president has made clear — and as all of you prove every single day — America first does not mean America alone.”

The assurances from top administration officials carried a whiff of implied threat that allies must do their share to support the alliance and the symbolism of Pence and Pompeo delivering the message in Poland was unmistakable.

Poland is among only a handful of NATO nations meeting the alliance’s agreed target for defense spending — a sore spot for Trump, who has been heavily critical of allies for not devoting enough resources to their militaries — and the right-leaning government’s effort to draw additional U.S. forces to Poland is seen by critics as an end run around NATO decision-making.

NATO members are expected to spend 2 percent of their gross domestic product, a measure of a country’s economic activity, on defense and Poland plans to exceed that goal over the next decade, while expanding purchases of U.S.-made weaponry and encouraging more U.S. forces to be based in the country. Pence formally closed a deal Wednesday for Poland to buy 20 sets of American mobile rocket launchers.

Pompeo visited U.S. and Polish troops serving together at a base some 44 miles from the Russian border, and said the United States is reviewing its troop presence in NATO and contemplating an increase on the “eastern flank.”

“As we enter the fifth year of Vladimir Putin’s war on Ukraine — a war he launched on European soil — we take seriously those concerns that Russia may one day try to open a front along the line right here,” Pompeo said before traveling to Warsaw, where he and Pence will attend a Middle East conference Thursday.

Pence and Shanahan, who is attending a meeting of NATO defense ministers at the alliance headquarters in Brussels, both repeated the United States’ commitment to mutual defense under NATO’s Article 5 — an attack against one member is considered an attack on all members — at a time when the alliance is facing a growing threat from Russia.

Trump broke with past U.S. presidents by omitting a pledge to common defense from his first address to NATO leaders in 2017. A changing cast of top U.S. national security officials has sought to reassure allies and project solidarity against Russia ever since.

“No matter what threats the future holds, you can be assured: Under President Trump, the United States will never waver in our commitment to Article 5,” Pence said.

He quoted Trump, from a 2017 address in Poland, saying, “ ‘the West will never, ever be broken — our values will prevail — our people will thrive — and our civilization will triumph.’ ”

“This is our cause,” Pence said. “This is why NATO exists. And it is why the United States stands with the Polish people — today and every day.”

The United States and Poland are also jointly hosting an international conference on Middle East peace and security Thursday that further highlights fissures between Washington and traditional allies in Western Europe that oppose the Trump administration’s withdrawal last year from the international nuclear deal with Iran.

Fearing the session is a U.S.-driven effort to paint Iran in a poor light as it marks the 40th anniversary of the Islamic revolution, many nations are sending relatively low-level representatives or staying away.

Poland’s right-leaning government, seeking to solidify ties with Trump, was an eager host.

Duda, who appeared to be only half joking when he proposed last year that Poland erect “Fort Trump,” several times on Wednesday referred to his hope that the Pentagon will significantly add to the approximately 4,500 U.S. troops based in ­Poland.

U.S. defense officials have been discussing increased rotational troop deployments to Poland and while no final decision has been announced, it is expected to elevate the troop level by at least around 1,000 service members.

Shanahan on Wednesday attended his first NATO defense ministerial since becoming Pentagon chief. He replaced Jim Mattis, the former Marine general who was revered in European capitals for his impassioned defense of America’s commitment to NATO and transatlantic partners.

Mattis resigned suddenly in December over differences with Trump, including over the two men’s views on historical U.S. alliances.

Speaking alongside NATO Secretary General Jens Stoltenberg after his arrival, Shanahan said he would seek to expand discussions about how NATO would meet the world’s evolving security challenges. He also said he would address how NATO can best employ increasing contributions from member states.

Stoltenberg, speaking later to reporters following an initial ministerial meeting, said Shanahan’s message was “very well received by all members because it was a very strong and very clear message about the U.S. ironclad commitment to NATO, to Article 5 and also the very strong personal commitment he has to NATO.”

NATO defense ministers said their first encounters with Shanahan were drama-free.

“He stated that the United States remains committed strongly to the alliance,” said Estonian Defense Minister Jüri Luik. “The signals from the U.S. have been very consistent. Nobody feels any degree of drama or surprise.”

Ryan reported from Brussels. Michael Birnbaum in Brussels contributed to this report.