A slowly regenerating Russian army is making incremental gains in eastern Ukraine against valiant but under-equipped Ukrainian forces. The United States and its allies are racing to deliver the enormous quantities of weaponry the Ukrainians urgently need if they are to hold the Russians at bay.

Both sides are fighting furiously, both sides are suffering heavy casualties, and for both sides it has become a race against time.

If the Ukrainians can hold out long enough for the new weaponry to arrive, there is a good chance they cannot only reverse Russia’s gains but inflict a decisive defeat that could inhibit Russian ambitions in Europe for years, analysts and U.S. and Western officials say.

The Russians are under pressure to make gains before the new weapons arrive and before their own exhausted troops and depleted armor reach the limits of their capacity to advance. The Russian military is in the process of reviving and resupplying their forces, resurrecting units that were depleted or destroyed in the first weeks of fighting, and slowly but steadily funneling them into eastern Ukraine.

The Russians are also stepping up missile strikes against fuel and ammunition storage depots and critical infrastructure such as railway lines used for the delivery of weapons. A growing shortage of fuel across Ukraine has stirred concerns about its ability to sustain supplies of fuel to the front lines.

“The question is, can we crush the Russians before the Russians get back on their feet?” said Ben Hodges, a former commanding general of the U.S. Army in Europe. “If we are not able to deliver enough of the things Ukraine needs to disrupt and destroy Russian artillery, Russian rocket fire and Russian forces before the Russians complete their reconstitution, then this could drag on for a very long time.”

“Then they’ll consolidate and wait for us to lose interest,” he added.

By aiming for a Ukrainian victory, the United States and its allies are casting a vote of faith in the Ukrainian military, whose performance has far exceeded initial expectations, as well as recognizing that Russia’s army is far less capable than had been assumed. It’s a major strategic shift from the first weeks of the war, when the Biden administration was making plans for a Ukrainian government-in-exile to be based in Warsaw.

The goal now is what Defense Secretary Lloyd Austin called a “weakened” Russia, one that won’t be able in the future to “do the kinds of things that it has done in invading Ukraine.”

But first the supplies of weaponry, notably long-range artillery, have to be delivered, and the Ukrainians have to be trained to use new Western systems, a process that is underway but will take weeks more. The United States and its allies are speeding up the deliveries they’ve promised. But transferring them from Eastern Europe into Ukraine is going to require an unprecedented logistical effort at a time when the main supply lines are increasingly being targeted by Russian missiles, Hodges said.

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New Russian tactics for new terrain

In recent days, the pace of what the Pentagon has described as “anemic” Russian advances of about a mile a day has slowed. The Ukrainians, meanwhile, are retaking territory in some areas, notably around the northeastern town of Kharkiv, where military officials said Thursday they have launched a counteroffensive.

Russia has not yet demonstrated it can overcome the multiple shortcomings that thwarted its attempt to seize Kyiv, including logistics problems, poor command and control and the low morale of its troops, officials say.

But the Russians have adapted their tactics to the open, flat terrain of the Donbas region, which gives them an advantage over the nimbler but less heavily armed Ukrainian military. The Russians’ slow pace appears to be a deliberate effort to mitigate the heavy casualties they suffered in the first weeks of the war, when armored columns surged down tree-lined roads and became easy prey for Ukrainian ambush teams.

Now the Russians are standing back from Ukrainian lines, pounding towns and villages with artillery, then moving in when the Ukrainians are forced under withering fire to withdraw.

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In some instances, the Russians are then abandoning the villages and the Ukrainians are simply moving back in, U.S. officials say. With territory changing hands on a daily basis along a 300-mile front line, it is hard in some places to discern which side has the advantage.

It’s a punishing, scorched-earth warfare that takes a heavy toll on civilians. Videos posted on pro-Russian social media show Russian forces moving into destroyed, depopulated villages, rendered uninhabitable by the force of the fire rained down on them.

It’s also taking a toll on the Ukrainian military, which is starting to feel the strain of more than two months of continual fighting on multiple fronts, Ukrainians have acknowledged.

The Ukrainians have refrained from issuing their own casualty figures, but for the first time they are admitting that they are suffering heavy losses.

The Russians are losing soldiers at a higher rate than the Ukrainians, “but we are not super people. We have casualties as well,” Oleksandr Danylyuk, a defense and intelligence adviser to the Ukrainian government, said in an interview.

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Burning through huge amounts of everything

The Ukrainians’ most urgent need is heavy caliber, long-range artillery to let them strike deep behind Russian lines, he and other military experts say. The first of the 155 mm Howitzers promised by the United States have reached the front lines and are being used, Austin told a Senate hearing on Tuesday.

Further U.S. deliveries will include Humvees, M-113 armored personnel carriers, Mi-17 helicopters, along with hundreds of thousands of rounds of ammunition, and other allies have pledged additional supplies.

The fighting is so ferocious that Ukraine is burning through huge amounts of everything it needs, from ammunition to armored vehicles, drones and fuel, Danylyuk said. “We are so far from actually reaching our needs it is difficult even to comment,” he said.

The Russians have “changed their strategy to a much better one. They’ve started treating Ukrainian forces as a serious opponent, which is not good for us,” he added. “Our troops still have superiority in terms of professionalism and knowledge of the terrain. But the price of that success is very high.”

Ukraine’s backers are watching anxiously yet hopefully. “Probably, these hours for the Ukrainian army are the darkest and most decisive hours of all,” said Kusti Salm, the permanent secretary in the Ministry of Defense of Estonia, which has been one of Ukraine’s staunchest supporters.

He said he is confident the Ukrainians “can hold the front.” Then, once the weapons arrive, he said, “there will be a turning point where it will become easier for the Ukrainians to achieve their goals.”

In some ways, they already have. By forcing the Russians to undertake a humiliating retreat from the battle for Kyiv, the Ukrainians have secured their capital and their sovereignty, and the war has turned into a fight over how much Russia will have to show for its effort to capture the country, said Rob Lee, a former U.S. Marine who is now with the Foreign Policy Research Institute.

In narrowing their goals to the Donbas area and the southeast of Ukraine, the Russians are able to bring greater force to bear in those places than they did when they were still fighting around Kyiv. The Russian force, though heavily depleted by massive casualties in the first weeks of the war, still has offensive capability. If the Russians are able to continue to amass forces, it is likely they will make further gains, Lee said.

But Russia also will eventually run out of personnel, said Phillips O’Brien, professor of strategic studies at Scotland’s St Andrews University. There is speculation that President Vladimir Putin will announce a mass mobilization that would potentially bring hundreds of thousands of recruits into the army. But it would take up to a year to train and equip them for the battlefield, O’Brien said.

“They have to get what they want and hold it relatively soon,” he said. “In the immediate period the Russians have more force. At some point the Ukrainians will start redressing that balance as they receive supplies.”

U.S. and Western officials have not clearly defined what a Ukrainian victory would look like, but Austin and other U.S. officials have repeatedly said that Ukraine “can win,” while stressing that the coming weeks will be critical.

British Foreign Secretary Liz Truss stated last month that the ultimate goal is to eject Russia entirely from Ukraine, including all the areas it captured in 2014, but most military experts think that is not feasible. A more achievable outcome would be to roll Russian lines back to where they were when Russia launched the February invasion, but that also will be tough, experts say.

“What’s important is that Putin is seen to fail,” said a Western official, who spoke on the condition of anonymity to discuss sensitive issues. “What that looks like might vary.”