With anger in Europe mounting over President Trump’s perceived abandonment of the continent, President Emmanuel Macron of France gathered the chiefs of staff of more than 30 armed forces on Tuesday to review the formation of a multinational peacekeeping force to monitor any cease-fire in Ukraine.

A brief statement from the French presidency at the end of meeting said the chiefs of staff reached an agreement that any force dispatched to Ukraine must be credible and conceived for a long-term commitment, must give unfailing support to the Ukrainian army and should not be disconnected from NATO and its “capacities.”

Mr. Macron said “it was the moment for Europe to exert its full weight, for Ukraine and for itself.”

The countries represented at the meeting, convened in partnership with Britain, were mainly European but included Japan, Canada, Turkey and New Zealand. The broad attendance reflected a widespread dismay at Mr. Trump’s “pause” in American military aid to Ukraine and his embrace of the views of President Vladimir V. Putin of Russia. Soon after the meeting, the United States said it would lift the pause on intelligence sharing and resume military assistance to Ukraine.

“I think the moment is important and your presence here sends out a real signal,” Gen. Thierry Burkhard, France’s military chief of staff, told the gathered officials at the start of the meeting, according to footage released by the French military

There is near unanimity in Europe that Ukraine is its front line against Moscow and that the defense of Ukraine equals the defense of the continent. There has been a sea change. A Europe that was long content to enjoy a post-Cold War peace dividend is now in a bristling mood of rearmament.

“Whatever it takes” were the words this month of Germany’s incoming Chancellor, Friedrich Merz, to describe “the rule for our defense,” given the change in American strategy. It was a powerful declaration of emancipation for a German Republic effectively created and shaped by the United States after World War II.

“Who can believe that the Russia of today will stop at Ukraine?” Mr. Macron asked in a speech to the nation last week. “As I speak, and for years to come, Russia has become a threat to France and to Europe.”

The French defense minister, Sébastien Lecornu, added on Tuesday that the new challenge facing Europe was “not so much the Russian threat as above all the unpredictability of our American partner.”

How and where any European peacekeeping force would deploy in Ukraine is unclear. Any truce, let alone peace agreement, appeared remote as a Ukrainian delegation met United States envoys, including Secretary of State Marco Rubio, in Saudi Arabia. Sergey V. Lavrov, the Russian foreign minister, said last month that any such European deployment was “clearly unacceptable.”

President Volodymyr Zelensky of Ukraine, convinced that Mr. Putin cannot be trusted, has spoken of the need for a security force of 200,000 troops, but that appears far beyond Europe’s capacity. A deployment in the low tens of thousands seems more plausible, designed to be a credible deterrent, perhaps complemented by an air force contingent to enforce a no-fly zone.

“I would see perhaps 15,000 to 20,000 European troops, enough to deter, but not so large as to be seen by Moscow as a NATO battle corps,” said Camille Grand, a defense expert at the European Council on Foreign Relations in Brussels. “You will want three lines of defense — first the Ukrainian army, second this European force of reassurance and over the horizon a capacity to reinforce that with air power.”

Mr. Trump waved away serious consideration of security during a televised meeting with Mr. Zelensky at the White House last month that veered into an angry confrontation.

“We’ll make a deal, and when the deal is made, I don’t think we talk about security,” Mr. Trump said, suggesting Mr. Putin could be trusted.

Whether the European force is conceivable, given Russian hostility and the possible lack of American support in critical areas, including intelligence, is an open question. The possibility that such troops would be drawn into the conflict if targeted by Russia, despite Mr. Macron’s pledge that they “would not engage in frontline combat,” is another complicating factor.

Beyond the narrow question of deployment, Europe has embarked with urgency on the quest to build a credible European army, a discussion that will continue Wednesday in Paris among defense ministers from France, Britain, Germany, Italy and Poland. Ukraine’s defense minister will join virtually.

How to fight together, accelerate munitions production and build readiness are core questions. Replacing what the United States provides — including intelligence capabilities, space capabilities, strategic transport and air-to-air refueling — amounts to an immense undertaking.

But the need for a sharp change of direction is acutely felt. Across Europe, mockery of Mr. Trump, combined with anger and astonishment, has steadily spread, even if the president has support among far-right parties and people tired of the domination of liberal elites.

A virulent speech this month by a centrist French senator, Claude Malhuret, has gone viral in Europe and the United States, with millions of views.

“We were at war with a dictator,” Mr. Malhuret said, alluding to Mr. Putin. “Now we are fighting a dictator supported by a traitor.”

He suggested that Mr. Trump had treated Mr. Zelensky like “a stable boy” and described Mr. Trump’s policies as “more than an illiberal drift, a beginning of the confiscation of democracy.”

“Washington has become Nero’s court,” he declared, alluding to the tyrannical Roman emperor, “with an incendiary emperor, submissive courtiers and a buffoon on ketamine in charge of purging the civil service.” The “buffoon” was an apparent reference to Elon Musk.

In subsequent interviews, Mr. Malhuret said he had expressed what Americans appeared afraid to say in the growing climate of fear that Mr. Trump’s first weeks in office had provoked. Certainly, he captured sentiments that are propelling Europe toward a military rebirth.