Without billions of dollars’ worth of American-made weapons, Ukraine’s forces may not last long against Russia.

However, the time Ukraine has before facing defeat depends on how quickly Europe and Ukraine can replace the artillery, missiles, air-defense systems, and other arms that the Trump administration halted.

The United States had planned to deliver up to $11 billion worth of weapons and equipment to Ukraine this year, with some coming from Pentagon stockpiles and some from new defense contracts, according to a new analysis by the Center for Strategic and International Studies in Washington. A former senior U.S. defense official said the actual amount was likely closer to $9 billion.

Although European countries have pledged unwavering support for Ukraine, which has only intensified since the Trump administration began decreasing aid, it would be nearly impossible for them to quickly fill the weapons gap. European defense industries have increased production but only in fits and starts, and individual countries need to maintain their own weapons stockpiles.

Europe cannot replace American aid in the military support of Ukraine, according to Lt. Gen. Ihor Romanenko, former deputy of Ukraine’s military general staff.

Ukraine itself has been producing drones and developing its own artillery systems, and intends to spend 26 percent of its budget on defense this year. However, top Ukrainian officials say that if American support is not restored, the military will be in dire straits.

Ukraine likely has a safety margin of six months without systematic assistance from the United States, but it will become much more difficult, according to lawmaker Fedir Venislavskyi.

Some analysts believe this timeframe may be overly pessimistic. Mark F. Cancian, one of the authors of the Center for Strategic and International Studies study and a former White House weapons strategist, predicts that Ukraine’s forces would begin to buckle within four months due to a lack of munitions and equipment to replace losses.

Why can’t Europe fill the gap?

Out of the $136 billion in military aid provided to Ukraine by allies from the start of the full-scale Russian invasion in February 2022 to the end of last year, almost half came from the United States, according to the Kiel Institute for the World Economy, a German research organization.

The American share has decreased over time as Ukraine and Europe’s defense industries have accelerated production. Currently, only about 20 percent of military hardware supplied to Ukraine comes from the United States, according to recent estimates by the Royal United Services Institute, an analytical group affiliated with the British military.

“However, this 20 percent contains the most lethal and essential equipment,” said Malcolm Chalmers, the institute’s deputy director general. The effect of a decrease in American arms is expected to be cumulative, not an immediate collapse for Ukraine.

The United States, being the world’s largest economy, has more resources at its disposal. For example, its Air Force has 17 large electronic surveillance aircraft, while Britain has only three.

The European Commission’s president, Ursula von der Leyen, announced a $841 billion plan to increase defense budgets across Europe and encourage joint procurement among states to speed up weapons manufacturing. But previous efforts have been unsuccessful due to E.U. countries prioritizing domestic spending and defense contractors struggling to produce large quantities of expensive weapons without upfront capital.

Ms. von der Leyen acknowledged the challenges in increasing European defense production.

“The real question is whether Europe can act as decisively as the situation requires and whether Europe is ready and able to act quickly and with the necessary ambition,” she declared.

While Europe’s artillery production is now close to meeting wartime demands, the production of more advanced weapons, such as air defenses crucial to Ukraine’s survival, still struggles to meet the quickly escalating demands.

Manufacturers cannot invest in these improvements without long-term contracts, which some governments have been unwilling to provide.

According to Mr. Grand, it would require more political will in Europe to forge defense contracts: “Money alone cannot solve everything.”

What is Ukraine doing to arm itself?

Ukraine’s prime minister, Denys Shmyhal, claimed on social media that the military and government have the means to maintain the situation on the front line. However, he did not disclose the country’s remaining stockpile, likely to prevent exposing vulnerabilities to Russia.

Mr. Shmyhal stated that Ukraine would be able to produce enough artillery by the end of the year and was manufacturing its own armored vehicles and anti-tank weapons. In 2024, Ukraine plans to increase its production of first-person-view drones.

Ukraine is also reportedly attempting to develop air defenses as advanced as the American-made Patriot system, which can intercept ballistic missiles. However, each system costs $1 billion and takes up to two years to build.

‘Doomed?’

In his study titled “Is Ukraine Now Doomed?”, Mr. Cancian predicted that without U.S. military aid, Ukraine would be forced to accept an unfavorable cease-fire agreement with Russia. This could involve ceding a fifth of its territory and giving up its hopes of joining NATO.

Some allies may also decide to cut back their own aid, reasoning that without American support, Ukraine’s situation is hopeless.

It is unclear whether the Trump administration will also halt intelligence sharing with Ukraine and whether Ukraine will lose access to the Starlink satellite internet system, owned by Elon Musk, Mr. Trump’s close ally.

However, it is clear that halting security assistance will only make it more challenging for Ukraine to achieve a just and lasting end to the war.