President Volodymyr Zelensky of Ukraine has been promised billions of {dollars} in new army support throughout a whirlwind tour of European allies that displays a placing shift within the political panorama, as Europe takes a extra central function in arming Ukraine for its anticipated counteroffensive to drive Russia from its territory.

Mr. Zelensky’s four-country tour started in Italy on Saturday and resulted in Britain on Monday, when Prime Minister Rishi Sunak pledged to supply a serious bundle of air-defense missiles and assault drones. That was along with Britain’s latest supply of long-range cruise missiles. And it adopted Germany’s pledge to offer Ukraine an almost $3 billion package of weapons, in addition to much less concrete guarantees of further weapons from France and Italy.

Europe’s present of help for Ukraine, analysts mentioned, underscores that the battle is in a pivotal part, with Ukrainian forces massing for a counteroffensive that might set the phrases for any future negotiation with Russia. It additionally displays a recognition that help for Ukraine within the United States, nonetheless by far the most important provider of weapons, is prone to come beneath stress.

Mr. Zelensky is involved that because the American presidential race heats up, President Biden will probably be much less in a position to lead diplomatic efforts, German and Ukrainian officers mentioned. Should a Republican win the White House, officers fear there will probably be a collapse in management amongst Western allies.

“This is a crucial moment in Ukraine’s resistance to a terrible war of aggression they did not choose or provoke,” Mr. Sunak mentioned in a press release, as he welcomed Mr. Zelensky at Chequers, the British chief’s nation residence outdoors London.

Mr. Zelensky, who hugged Mr. Sunak after climbing out of a helicopter, described the conferences as “substantive negotiations” and referred to Mr. Sunak in a publish on Twitter as “my friend Rishi.” The two met in a room utilized by Winston Churchill to write down his rally-the-nation radio broadcasts throughout World War II.

Mr. Zelensky held equally heat conferences with Chancellor Olaf Scholz of Germany in Berlin and President Emmanuel Macron of France in Paris, each on Sunday; and with Prime Minister Giorgia Meloni of Italy in Rome, the place he additionally met Pope Francis, on Saturday. From his visits, Mr. Zelensky emerged with at the least the promise of extra weapons.

The new long-range missiles, assault drones and tanks and different armored automobiles secured from allies in latest days will fulfill many, however not all, of the calls for for weapons that Ukraine has mentioned it wants for a counteroffensive.

“The trajectory across Europe is to do more,” mentioned Malcolm Chalmers, the deputy director basic of the Royal United Services Institute, a analysis group in London. “That is partly about the evolution of European politics. It’s also driven by the expectations that the next few months are critical, and things could go either way.”

“The prospect of faltering American commitment as we approach the election is at the back of people’s minds,” Mr. Chalmers added. “That has led to the desire to get Ukraine to the best possible position before negotiations.”

White House officers have mentioned privately that they continue to be assured they’ve bipartisan help in Congress to proceed serving to Ukraine within the close to time period, however they haven’t sought further monetary support since Republicans took over the House and, so, haven’t examined that assumption.

Taken collectively, army analysts mentioned, the European reinforcements make it extremely seemingly that Ukrainian troops will quickly strike again at Russian forces that management the nation’s south, setting off what might be one of many bloodiest stretches of the 15-month battle. Mr. Zelensky had warned that with out extra weapons, the counteroffensive might be delayed.

Germany was the primary to reply his name: Its dedication included 30 further Leopard tanks, 20 armored preventing automobiles, 16 air-defense techniques, greater than 200 drones and a slew of different arms and ammunition.

The Leopards and armored preventing automobiles might be helpful in reclaiming territory, because the grassy steppe in Ukraine’s south is suited “for tank or maneuver warfare,” mentioned Jacob Funk Kirkegaard, a former Danish military intelligence officer who’s now a senior fellow on the German Marshall Fund in Brussels.

Britain’s missiles and drones, he added, might be used to assault Russian bases in Crimea, the Black Sea peninsula that Moscow seized after a disputed referendum in 2014.

Russia’s Defense Ministry claims that the long-range missile, often known as Storm Shadow, has already been fired by forces in japanese Ukraine, injuring six civilians on Saturday. The declare couldn’t be independently verified, and there was no remark from Britain.

The Kremlin expressed anger at Britain’s pledge. “We take an extremely negative view of it,” mentioned the Kremlin spokesman, Dmitri S. Peskov, in line with the Russian state news company TASS. But he added that the weapons would “not have any significant impact” on the battle.

The United States has resisted sending long-range missiles to Ukraine, partially to keep away from escalating the battle with weapons that might attain into Russian territory. That can also be one cause the Biden administration has not acceded to Ukraine’s requests for American-made F-16 fighter jets.

American officers have mentioned, nevertheless, they won’t block different states from sending their very own weapons to Ukraine. And analysts mentioned Britain wouldn’t have given the long-range missiles with out Washington’s tacit approval.

Mr. Sunak promised to begin coaching Ukrainian fighter pilots on F-16s this summer season. Mr. Macron additionally mentioned in a televised interview on Monday that France and different European nations had “opened the door to training pilots” and that “training can start right away.” He added that “discussions are underway with the Americans.”

Europe’s elevated pledges, analysts mentioned, replicate a rising confidence that Ukraine’s troops may make decisive breakthroughs with its counteroffensive. Driving Russia out of Ukrainian territory would enormously enhance Mr. Zelensky’s leverage in any negotiation with President Vladimir V. Putin of Russia.

“We wouldn’t be committing this amount of weapons to Ukraine at this point if the thinking was that it was not likely that they would succeed,” Mr. Kirkegaard mentioned. “Nothing is certain, but we in the West seem to be increasingly bullish on Ukraine.”

In February, on his first go to to Western Europe after the battle broke out, Mr. Zelensky visited London, the place he appealed for fight plane earlier than touring to Paris for a gathering with Mr. Macron and Mr. Scholz. This time, Britain was Mr. Zelensky’s ultimate cease.

With its aggressive strategy, Britain has usually acted as a catalyst for extra reluctant Western nations to provide Ukraine with heavier weapons. Its determination to ship a squadron of Challenger 2 battle tanks, for instance, foreshadowed choices by Germany and the United States to ship extra subtle tanks.

Britain additionally supplied about $2.8 billion in army support to Ukraine in 2022, making it the nation’s largest backer after the United States and Germany.

European and Ukrainian officers mentioned Mr. Zelensky was additionally privately pushing for a larger European function on the diplomatic stage. Ukraine is cautious of nations with shut relations to Russia, like China and Brazil, drawing up peace plans or performing as mediators.

Mr. Zelensky hopes to get allies like Germany extra concerned, in line with a Ukrainian official who spoke on the situation of anonymity due to the sensitivity of the matter.

Mr. Macron, who has tried to place himself as a key diplomatic participant in future peace talks, pressed China’s high chief, Xi Jinping, final month to make use of his shut relationship with Moscow to assist begin negotiations.

China later introduced it will ship Li Hui, its particular consultant for Eurasian affairs, to Ukraine after a telephone name between Mr. Xi and Mr. Zelensky. Mr. Li was to start his journey on Monday to Ukraine and Russia in an try to assist negotiate an finish to the battle.

Reporting was contributed by Lara Jakes in Rome, Erika Solomon and Christopher F. Schuetze in Berlin, Peter Baker in Washington, Constant Méheut in Paris and Vivian Wang in Beijing.