On the face of it, President Trump is right to try to end a destructive war in Ukraine that is now approaching the three-year mark, and to open talks with Russia. This war has to end, and at this stage it can end expediently only through negotiations.

Yet the way the president is going about this vital task is misguided, counterproductive and unfair to Ukraine.

In quick order, Mr. Trump has sent his lieutenants to negotiate with Russia without the participation of either Ukraine or the NATO allies who are most directly threatened by a resurgent Russia; he has cruelly suggested that Ukraine — not Russia — is responsible for the war by not making some unspecified “deal” at the outset; he has demanded access to Ukraine’s mineral wealth as the price of continuing military support; and he has baselessly called Volodymyr Zelensky, the Ukrainian president, a “dictator without elections.”

In effect, Mr. Trump and his administration are pivoting U.S. foreign policy 180 degrees, pointing the way to a peace that would run counter to the American and Western mission of securing a sovereign, independent, democratic and prosperous Ukraine. It is a reversal of the past three years — since Russia invaded its neighboring nation, strong majorities of Americans of every party and persuasion have supported Ukraine — but also a repudiation of a nearly eight-decade core belief that the United States was safer in a world where it stood against aggression and authoritarianism and for freedom.

Mr. Trump has long displayed an open admiration for the bullies of the world and a particular affinity for Vladimir Putin, and he long ago developed an antipathy for Mr. Zelensky, who was the reason for Mr. Trump’s first impeachment in his first term. But the facts are that Mr. Zelensky is not a dictator, and Mr. Putin is. And that is with whom Mr. Trump has now aligned our country.

These actions run counter to America’s ideals, interests and security — and to the values of Mr. Trump’s own party.

The volume of misinformation and betrayal here is breathtaking. The very act of discussing an end to the war without the presence of Ukraine or any other ally at the table violates the fundamental principle the United States has proclaimed from the outset of its support for Ukraine, “nothing about Ukraine without Ukraine.” That alone is a huge victory for Mr. Putin, who portrays his war as a struggle for spheres of influence among great powers rather than the brutal land grab that it is.

Republican members of Congress must see that. They surely realize how crucial America’s close alliances with Ukraine and Europe have been to promoting stability, maintaining national security, fostering economic growth and containing the spread of anti-democratic forces since World War II. Most of them have been outspoken supporters of Ukraine since the February 2022 invasion, sometimes assailing the Biden administration for doing too little. They know this is no time to abandon Ukraine, and they know who the villain is in this story.

Secretary of State Mario Rubio, leading the negotiations with Russia, was a full-throated critic of Mr. Putin during his time in the Senate, calling the Russian president a “killer.” The Senate majority leader, John Thune of South Dakota, was among 22 Republicans who voted a year ago to send $60 billion in aid to Ukraine against Mr. Trump’s wishes; before that, the senator had declared that “the only thing Putin responds to is strength, so the United States must send a strong and unified message that this aggression will not be tolerated.”

Yet today, while some Republicans have voiced dismay over Mr. Trump’s actions, there has been no concerted effort to challenge him. “Right now, you have to give him some space,” Senator Thune said at a news conference after a closed-door Senate lunch with Vice President JD Vance. On Mr. Trump calling Mr. Zelensky a dictator, he said only, “The president speaks for himself.”

No, he does not. He speaks for America, and for the Republican Party he leads. When he is wrong, it is the duty of his lieutenants and allies to tell him so.

Mr. Trump may be right that NATO allies have long underinvested in defense, and could have done more for Ukraine had they not. The Biden administration spent an estimated $185 billion on military support for Ukraine and all but exhausted U.S. military stocks, but never really articulated a strategy for ending the war. Trade imbalances do need to be addressed, and illegal migration is a major problem.

In short, there are, and always have been, challenges that the president must address, which, as the leader of the world’s most powerful and richest democracy, he is called upon to manage wisely and humanely, through diplomacy where possible and through judicious application of pressure or even force when necessary.

Mr. Trump, by contrast, has taken an unstudied, bullying and petulant approach to foreign policy, allowing consequential decisions to be determined not by the best interests of the nation but by his ego and grudges.

And, as Mr. Trump has waged a concerted and well-choreographed assault on the institutions of American government and the longstanding principles of its foreign policy over the past few weeks, Republican lawmakers and many other Americans who profoundly disagree with the president’s approach and worldview have done little but tried to stay out of the line of fire, even sometimes backing away from longstanding positions.

They are consoling themselves by saying that the new president needs time or “some space,” or that his petulant posts are not policy, or that the means are justified by some aspect of the administration’s blitzkrieg that they agree with. Or they have simply been cowed by Mr. Trump’s certain retaliation against any who defy him.

But for the president of the United States to dismiss the appalling sacrifices of the Ukrainians as their own fault, to belittle Ukraine’s president, to make deals without Ukrainian or NATO involvement, and to indulge in his admiration of the tyrant in the Kremlin goes far beyond any behavior that Americans should tolerate.