White House clash raises tough question for Ukraine: How long could it fight without US?

Now, the question is whether Europe has the firepower—and the political will—to help Ukraine hold off the Russians.

Without Washington at its side, Ukraine would have to lean more heavily on its European allies and its own domestic defense production. It could likely maintain its current fighting strength for at least a few months, officials and analysts say. After that, it could face shortages of ammunition and lose access to some of its most sophisticated weapons.

A parade of European leaders have expressed solidarity with Ukraine after the meeting in the Oval Office descended into an unusual on-camera clash between President Trump and Ukrainian President Volodymyr Zelensky. The European officials offered Zelensky and Ukraine their backing and said they would ensure Ukraine doesn’t stand alone.

“We stand with Ukraine for as long as it may take," U.K. Prime Minister Keir Starmer told Zelensky as he met him in Downing Street ahead of a meeting with European leaders taking place in London on Sunday. Starmer said he wanted to achieve a “lasting peace" in Ukraine and has also been in contact with Trump as he attempts to broker a compromise between the U.S and Europe. The U.K. said Saturday it was making a loan of £2.26 billion, or roughly $2.8 billion, to Ukraine to fund its military.

The coming weeks will show whether Europe can back up its rhetoric by stepping up support for Ukraine if Washington walks away.

Ukrainian soldiers train in the eastern region of Donetsk.U.S.-provided Bradley infantry fighting vehicles have played an important role in Ukraine’s defense.

Zelensky spoke with several other European leaders by phone after the White House meeting, including Starmer, French President Emmanuel Macron, NATO Secretary-General Mark Rutte and Antonio Costa, who convenes and plans meetings of European Union leaders.

European diplomats briefed on those calls said Zelensky showed an impressive level of calm after the tense exchange with Trump and Vice President JD Vance that stunned senior officials in Washington.

In the past, Zelensky has been skeptical of Europe’s ability to act alone to defend Ukraine. He has consistently said that the U.S. would be needed to deter the Kremlin from attacking his country again in the future if there were a cease-fire deal. He stressed that again on Friday in his meeting with Trump.

The U.S. has sent nearly $70 billion in military aid—dwarfing contributions from any of Kyiv’s other allies—since the start of the war, according to Zelensky.

But European nations have been steadily stepping up their support throughout the war. The EU and its member states have given over $50 billion in military assistance.

Last year, the EU, the U.K. and Norway combined gave Ukraine around $25 billion in military aid—more than the U.S. sent, according to European officials.

U.K. Prime Minister Keir Starmer, who met with Trump on Thursday, plans to host European leaders to discuss Ukraine this weekend.

Meanwhile, Ukraine has vastly expanded its domestic arms industry, producing $30 billion a year in weaponry, according to Ukrainian officials. Last year, the country produced 1.5 million drones, which have played an increasingly important role on the front lines, allowing Ukraine to hold off Russian forces with minimal casualties. Ukraine says it can produce 3,000 missiles and 30,000 long-range drones this year.

Former President Joe Biden, before leaving office, sent weapons from existing U.S. stocks and signed contracts with the U.S. defense industry to provide ammunition, air-defense interceptors and other materiel. It wasn’t immediately clear whether Trump could halt some of those supplies if he chose to do so.

Some advanced U.S. weapons, such as its air-defense systems and surface-to-surface ballistic missiles, will be impossible for Ukraine to replace in the short-term once supplies begin to run out. Europe doesn’t make enough of them.

“Even one year of fighting without the U.S. would be a major achievement, and a major rebuke for Trump’s shortsighted approach," said Mykola Bielieskov, a senior analyst at Come Back Alive, a Ukrainian charity that has supplied drones to the military. He added that it seemed as though Trump had been “looking for an excuse, if they indeed plan to cut aid to zero."

European officials said the clash between Zelensky and Trump in Washington on Friday underscored the urgency of accelerating work to aid Ukraine.

On Sunday, European leaders will gather in London to discuss security guarantees that countries are prepared to offer if there is a cease-fire deal. On Thursday, EU leaders meet for an emergency summit on Ukraine and the continent’s defense vulnerabilities. Two weeks later, they will gather again in Brussels to make or sign off decisions.

Ukraine’s domestically produced drones are already a key part of its arsenal.Front-line cities such as Pokrovsk continue to bear the brunt of Russia’s advance in eastern Ukraine.

On Friday, Italian Prime Minister Giorgia Meloni, one of Trump’s closest European allies, called for a summit between the U.S. and Europe to discuss Europe and other security challenges.

The EU has already earmarked 30 billion euros, equivalent to about $31 billion, for Ukraine for this year, and some of that could be used as military aid. But much of that sum is expected to go toward budget assistance that Kyiv needs to pay salaries and keep basic services going as well as potential military aid.

Some European diplomats say the EU should commit to at least matching last year’s 20 billion euros in military assistance for Ukraine. But reaching agreement on that ahead of the meeting in Brussels in two weeks will be a challenge, given the need for most EU leaders to back such a plan.

A number of European countries have come out with specific military aid pledges for Ukraine, including Sweden, Denmark, Poland and Spain. Britain and France have already penciled in billions in additional assistance for 2025.

Pressure is rising in Europe to seize nearly 200 billion euros in frozen Russian assets sitting mainly in Belgium to bolster support, but the idea still lacks backing from Europe’s most powerful countries.

“Today, it became clear that the free world needs a new leader," the EU’s foreign policy chief, Kaja Kallas, said on X on Friday. “It’s up to us, Europeans, to take this challenge."

Tymofiy Mylovanov, president of the Kyiv School of Economics and Ukraine’s former economic development minister, said that Europe had the capacity to help Ukraine—including, potentially, buying American weapons to send to Kyiv.

But he said it was an open question whether European leaders would continue standing with Ukraine if Trump started pressing them to back away. He noted that Trump had leverage over European leaders, and he could potentially threaten, for example, to cut them off from U.S. intelligence

“Right now, Europe is showing solidarity and support," Mylovanov said. “They’re less independent from Trump than they think."

Write to Laurence Norman at laurence.norman@wsj.com and Ian Lovett at ian.lovett@wsj.com