The America-sized hole in Ukraine’s war effort

The superior accuracy and greater range of M777 howitzers, supplied by the U.S., hit back against Russia’s mostly Soviet-designed artillery. Then U.S. intelligence tipped off Ukraine’s generals that Russia had moved several battalions to another front. Accurate U.S.-made rocket artillery struck Russian fuel depots and weapons stores, leaving the Russian army short of supplies ahead of a rapid Ukrainian counteroffensive that retook dozens of towns.

Now, with Russia’s military again grinding its way forward, the Trump administration has halted weapons deliveries and intelligence sharing that have been critical to Ukraine’s resistance against a three-year invasion by its giant neighbor that has killed tens of thousands and razed dozens of cities.

The impact of the halt—which U.S. officials described as a temporary pause designed to force Ukrainian President Volodymyr Zelensky to negotiate with Russia—will be limited at first, but is likely to grow over time as Ukraine’s stocks of U.S. ammunition run down, and weapons systems cannot be repaired or replaced, officials and analysts said.

The main immediate impact will be felt from the cutoff of intelligence, which will constrain Ukraine’s ability to accurately target long-range strikes to knock out Russian troops and equipment before they reach the battlefield. On Friday, Maxar Technologies said that the U.S. has cut off Ukrainian access to the satellite images that the company supplies through an American government program—imagery used by Ukrainian forces to plan and mount operations, particularly using explosive drones.

A lack of fresh arms deliveries will take longer to have an impact but the effect would be broader. A dwindling stock of long-range air-defense missiles would erode Ukraine’s ability to intercept Russian ballistic missiles targeting cities. A lack of spares could see its top-performing infantry fighting vehicles and howitzers fall out of use. Although the immediate reaction from front-line soldiers was one of weary resilience, morale is likely to suffer as the withdrawal of support from Ukraine’s main backer saps confidence.

“These impacts will cascade and compound," said a U.S. official.

“Russia is using missiles and weapons from all over the world, from North Korea to Iran, to fire from all directions into Ukraine. But we have limited Ukraine in using our weapons to fire back," said retired Air Force Gen. Philip Breedlove, who served as the top NATO commander from 2013 to 2016 and has been a strong supporter of Ukraine.

Top U.S. officials are meeting on Tuesday in Saudi Arabia with their Ukrainian counterparts to set the stage for potential peace talks between Ukraine and Russia, raising the possibility that the pause in assistance could end soon.

Whatever happens, leaders in Kyiv said that Ukraine will keep up the fight. The Biden administration increased supplies of ammunition ahead of Trump’s inauguration in anticipation of a potential cutoff. Ukraine makes more than half of its own weaponry, including game-changing drones. European Union members, which along with the U.K. have roughly matched U.S. military support to Ukraine since Russia’s 2022 invasion, met Thursday to agree on a surge in military spending that could allow them to offer more support to Ukraine.

“It’s no longer 2022," Zelensky said in a video address Tuesday. “Our resilience is higher now. We have the means to defend ourselves."

The Kremlin has welcomed the pause to U.S. deliveries, which came just as Ukraine has significantly slowed Russian advances in recent weeks. Russian President Vladimir Putin is far short of his ultimate goal of subjugating Ukraine and is struggling to achieve his shorter-term objective of occupying Ukraine’s two easternmost regions.

Ukraine’s defenses have inflicted heavy losses on Russian forces as they have nibbled off small pieces of land along the 800-mile front lines.

Ukraine’s defensive strategy relies on eliminating Russian troops and weaponry before they reach Ukrainian front-line trenches, which are manned by increasingly exhausted and thinned-out infantry.

More than half of front-line strikes are carried out by small explosive drones manufactured in Ukraine, which accurately target Russian armored vehicles and infantry.

The use of drones has offset Russia’s artillery advantage. Russia has more guns and more shells, bolstered in recent months by supplies from North Korea.

The U.S. accounts for over half of Ukraine’s foreign supplies of the important 155mm artillery shells, according to one person familiar with the matter. Even with U.S. shells, Ukraine currently is able to fire one for every three Russia does, that person said.

“We already don’t have parity with Russia in weapons and ammunition," said Ukrainian Army Lt. Dmytro Yanok, who commands an M777 battery. “If the U.S. ends ammunition deliveries for good, the situation will become much worse."

Franz-Stefan Gady, a Vienna-based defense analyst, said that could necessitate tactical withdrawals in some areas, although he believes the front can hold in the near term.

“Overall it’s not catastrophic," he said. “The dependence on the U.S. is much less in 2025 than it would have been in 2022 or 2023."

Ukraine’s military relies heavily on the U.S. for rockets and missiles that can strike accurately behind Russian lines. The U.S. provided High Mobility Artillery Rocket Systems, or Himars, to Ukraine in 2022 that fire Guided Multiple Launch Rocket System rockets, or GMLRS with a range of about 45 miles. GMLRS strikes have destroyed Russian command posts, ammunition dumps, fuel stores and groups of infantry. It is a lack of these rockets which perhaps will affect Ukraine most, followed by artillery shells and air-defense missiles, said a former senior U.S. official involved in supplying Ukraine.

Himars also launch longer-range ATACMS, which can shoot up to 186 miles. These ballistic missiles have been particularly effective at striking Russian airfields, command centers and supply lines in occupied Ukraine, and since November in parts of Russia.

The result of those strikes is to relieve pressure on front-line trenches as they reduce the quantity and speed with which Russia can deploy men and equipment to the battlefield.

“If they are not able to attrit the Russians before they come into the battle zones, then it just makes it harder on their army. They need to be able to kill Russians deeply and at range precisely," said Breedlove, the former top NATO commander.

During a rapid operation to seize a chunk of Russia’s Kursk province last fall, Ukraine used GMLRS to strike a column of Russian armored vehicles rushing to the battlefield and hit bridges to cut supplies to front-line troops.

Long-range strikes depend on U.S. intelligence, which helps determine targets and provide accurate location data, so the intelligence cutoff will affect strikes even before ammunition is depleted.

Ukraine was already running low on ATACMS at the end of last year.

“We’ve not seen a single confirmed ATACMS strike into Russia since Donald Trump was inaugurated," said George Barros, an analyst at the Institute at the Study of War, a Washington-based think tank.

The loss of U.S. ammunition would be compounded by the gradual degradation of weapons already supplied if they cannot be repaired or replaced.

The M777, a howitzer designed for the U.S. Army in the 1980s, is Ukrainian artillerists’ favorite weapon, said Col. Serhiy Musiyenko, deputy commander of Ukraine’s missile and artillery forces.

It can shoot up to 19 miles, compared with the 15-mile ranges of their Soviet artillery, and its accuracy gives Ukrainian gunners an advantage over the Russians, Musiyenko said.

Ukraine was using Soviet artillery until it received its first 12 M777s near the northeast Ukrainian city Izium in June 2022.

“We immediately felt the advantage," said Musiyienko, whose own combat experience stretches back to 2014.

More M777s have been sent to Ukraine, or around 180 pieces, than any other artillery. All but around 10 have come from the U.S.

While the M777 was designed and jointly made in the U.K., it was created for the U.S. Army and so its maker, BAE Systems, needs Washington’s approval to sell guns and parts abroad.

European nations later sent howitzers that were newer, could hit further and at a faster rate. They were also self-propelled so could escape counter fire faster than the towed M777 once they had fired.

But many Ukrainians still preferred their M777s because they were easy to use, rarely broke down and if damaged were quickly repairable. Among self-propelled artillery, Ukrainians like the U.S.-supplied M109 because its cab provides good protection.

Ukraine’s stock of M777s is on average off the battlefield for repairs half the time of the country’s more modern self-propelled artillery, according to one Ukrainian official.

A lack of spare parts could also eventually remove many Ukrainians’ favorite infantry fighting vehicles from the battlefield. The Bradley, an aging though highly mobile vehicle, can quickly shuttle troops to and from the front lines and protect them from drones and guided missiles.

The U.S. has supplied more than 300 Bradleys, which are no longer in production. Last year, U.S. officials said they aimed to provide three months of spares for equipment sent to Ukraine.

Ukrainian mechanics have already become adept at manufacturing their own spare parts. At one repair depot, around 20% of the parts that Ukrainian mechanics used to fix damaged M777s, including hoses and some gauges, were domestically produced.

Still, that doesn’t always work. The M777, for instance, is made of titanium, an ultralight metal that is hard to weld and not readily available. German officials have said that Ukrainian attempts to use foreign parts on their tanks and howitzers often ended up further damaging the vehicles.

Even with U.S. supplies, Capt. Oleksandr Shyrshyn from the 47th Brigade hasn’t always had enough spares for the Bradleys under his command. The Bradley is the best infantry fighting vehicle he has worked with, and it would be unpleasant if the lack of spares puts them out of action, he said. But there are not enough of them to have a critical impact on Ukraine.

“We have other means," he said.

Alongside more traditional weapons, the U.S. makes some of Ukraine’s most sophisticated and new equipment. Europe has supplied much of Ukraine’s short- and midrange air defense. But Europe currently has nothing on land that can compare to the U.S. Patriot system’s ability to destroy Russian ballistic and hypersonic missiles. Both missiles are so fast and ballistics so large that there are few defenses against them.

The Patriot’s success can be seen in Ukrainian government data. Between October 2023 and last November, Ukraine shot down only 10% of ballistic missiles, according to data from the Ukrainian Air Force. But those fired at Kyiv, where Ukraine has at least one Patriot system, were typically intercepted.

“Everyone wants a Patriot," said Viktor Petryshyn, a Ukrainian air defense commander in southern Ukraine. Petryshyn uses a Soviet S-300 system that has shot down Russian drones, missiles and planes.

Ukraine has around five Patriot systems, three of which come from the U.S. Ukraine’s biggest problem will now be getting the U.S. manufactured missiles.

“They were not getting enough as it was," said Nick Reynolds, a research fellow at the Royal United Services Institute in London.

“It would expose Ukraine’s critical national infrastructure even more and put the population to more risk," he said. Ukraine’s electricity grid has been targeted through much of the war, causing black outs and hurting the country’s economy.

Underscoring the problems, France and Italy supplied a Samp/T missile defense system that was designed to handle ballistic missiles. But after arriving in Ukraine, the system’s software struggled and it failed to hit ballistic missiles, according to people familiar with the matter. Ukraine also soon ran low on the Aster missile that the Samp/T uses, they said.

A spokesman for Eurosam, the joint venture that makes the system, didn’t immediately comment.

The intelligence cutoff could deprive Ukraine of early warnings about missile strikes that have helped defenders down missiles.

Russia launched a large-scale aerial bombardment of Ukrainian cities early Friday with 67 missiles and 194 attack drones, according to the Ukrainian Air Force. The majority were intercepted, the air force said, but officials reported damage to power and gas facilities. Early Saturday, Russia struck the eastern city of Dobropillya with two ballistic missiles, killing at least 11, Ukrainian officials said.

One of the key pieces of nonlethal U.S. technology that Ukraine uses is Starlink, the satellite-internet service developed by Elon Musk’s SpaceX. Starlink still provides effective and secure communications between troops and their commanders and allows feeds from drones to be shared to help artillery gunners target their fire.

Ukraine’s supporters supplied thousands of Starlink terminals to Kyiv. But SpaceX and Musk, who now has a senior U.S. government role and has become increasingly critical of Kyiv, can cut Ukraine off from the service.

Ukraine, meanwhile, is relying on innovation of its own.

Ukrainian companies are producing drones that fly on autopilot to strike a target selected by an operator, preventing Russian jammers from downing them.

A Starlink communication device on the front line during Orthodox Christmas in 2023.

Ukraine has also solved the conundrum of how to down ubiquitous Russian reconnaissance drones cheaply—by striking them with small explosive drones.

“Technology development dictates the battlefield, not America," said Taras Chmut, head of Come Back Alive, a charity supplying weapons to the Ukrainian army. “America has fallen behind modern warfare. Europe has fallen behind modern warfare. Ukraine is waging the war of the future."

Write to James Marson at james.marson@wsj.com, Alistair MacDonald at Alistair.Macdonald@wsj.com and Michael R. Gordon at michael.gordon@wsj.com