By Phil Stewart and Idrees Ali

WASHINGTON (Reuters) - U.S. Defense Secretary Pete Hegseth's attention-grabbing overseas debut may have irritated some key Republicans and alienated allies in Europe, where his statements on Ukraine and NATO went down like a lead balloon.

But his forceful comments gained him a nod from one key listener: U.S. President Donald Trump.

And Hegseth - who on Saturday wraps up a week-long trip to Belgium, Germany and Poland - delivered a message at the heart of Trump's America First agenda.

Speaking at the NATO headquarters in Brussels on Wednesday, Hegseth said a return to Ukraine's pre-2014 borders was unrealistic and the Trump administration does not see NATO membership for Kyiv as part of a solution to the war triggered by Russia's 2022 invasion.

Trump broadly backed Hegseth's remarks on Thursday on NATO membership, saying, "I think probably that's true," because, he said, Putin would not allow Ukraine to join the military alliance.

"I thought his comments were good yesterday, and they're probably good today," Trump said.

Not everyone was so positive.

"I don’t know who wrote the speech — it is the kind of thing Tucker Carlson could have written, and Carlson is a fool," Senator Roger Wicker, the Republican who leads the Pentagon's main oversight committee in the Senate, told Politico, referring to the conservative media personality who was once a Fox News host.

Wicker had championed Hegseth's nomination throughout a bruising confirmation review in which Democrats united against the nominee and three Republicans joined them, as questions were raised about Hegseth's qualifications, temperament and views about women in combat.

Asked whether he spent a lot of political capital getting Hegseth, a military veteran and former Fox News personality, confirmed, Wicker said: "I surely did, yes."

Republican congressman Mike Turner said issues like the future of NATO membership for Ukraine should not be taken off the table.

"We don't need members of the cabinet, President Trump's cabinet, to be defining those in the public," Turner said on CNN on Friday.

Republican congressman Don Bacon responded to Hegseth's comments by saying that there should be moral clarity on who started the war.

"There are consequences of rewarding the invader even if its leader foolishly led over 700,000 of its citizens to slaughter," Bacon said on X.

UNCLE SAM OR UNCLE SUCKER?

Trump has played down any tensions. On Friday, he said he had not seen Wicker's comments but would reach out to both him and Hegseth.

"Roger's a very good friend of mine, and, Pete is obviously, he's been doing a great job," Trump said.

Hegseth, in what some analysts saw as walking back his remarks, clarified on Thursday that Trump was the one who would ultimately decide what was on or off the table in the Ukraine talks.

Calling Trump the world's best negotiator, Hegseth said it would be not appropriate for him to "declare what President Trump will do or won't do, what will be in or what will be out."

He also delivered a message that resonates with Trump and his America First agenda: Europe has been taking advantage of the United States.

Hegseth spoke about Europe's reliance on costly U.S. military deployments for its defense. Trump has criticized Europe over what he sees as its unfair tariffs against American goods.

The United States and European Union have the world's largest commercial relationship, trading 1.5 trillion euros ($1.55 trillion) of goods and services in 2023.

Speaking to reporters at NATO headquarters, Hegseth said: "Make no mistake: President Trump will not allow anyone to turn Uncle Sam into 'Uncle Sucker.'"

In Trump's first term, NATO was seen as a red-line for his Pentagon chiefs. Jim Mattis, his first defense secretary, resigned in part because of Trump's skepticism towards NATO.

America's European allies were critical of Hegseth.

"I think that was clumsy. I think that was a mistake," Germany's Defense Minister Boris Pistorius said, also criticizing Trump.

European foreign policy chief Kaja Kallas went further.

"It is appeasement. It has never worked," she said.

(Reporting by Phil Stewart and Idrees Ali; Editing by Don Durfee and Rosalba O'Brien)