DOVER AIR FORCE BASE, Del. — Virtually every day, a line of 18-wheel trucks loaded with weapons or ammunition pulls up to a sprawling warehouse here nestled near an asphalt runway stretching nearly two miles. Drawn from U.S. military depots around the country, the lethal cargo is unloaded onto pallets that will be packed aboard cargo planes bound for Europe, the next stop on its journey to the front lines in Ukraine.

The constant tempo has evolved from choppy beginnings into precision choreography in the 10 months since Russia's Ukraine invasion. Similar scenes are being repeated at bases and seaports up and down the East Coast as U.S. commitments surpass $20 billion in military support for a war in which the United States, at least officially, is not a participant.

"It's all a steady flow on purpose," Air Force Master Sgt. Christopher Mitcham said this fall as he supervised the activity in Dover. "You just understand that you're at the mercy of what the mission needs."

Both the mission and its needs have undergone a radical transformation since Russia's full-scale invasion in late February, when the Biden administration provided minimal support for vastly outnumbered Ukrainian defenders. Since then, Washington has dug ever-deeper into its own arsenal and treasury to supply Kyiv with massive quantities of arms.

This week, the administration marked the historic visit to Washington by Ukrainian President Volodymyr Zelensky by announcing the transfer of a Patriot missile battery, the most sophisticated air defense system in the U.S. arsenal.

But the initial war supply operation clearly wasn't built for the long haul. As the grueling conflict continues with no end in sight, it has exposed flaws in U.S. strategic planning for its own future battles, and revealed significant gaps in the American and NATO defense industrial base. Stocks of many key weapons and munitions are near exhaustion, and wait times for new production of missiles stretches for months and, in some cases, years.

In interviews over the last several months, more than two dozen senior U.S., European and Ukrainian government and military officials and experts, some speaking on the condition of anonymity about the strategically and diplomatically sensitive effort, revealed new details about how a U.S.-led consortium of democracies has gone about keeping Ukraine afloat in the war.

For much of the past year, the United States and its allies have been playing catch-up in supplying Ukraine. Many of the systems now deployed there were initially withheld as too complicated for its forces to use and maintain, too liable to provoke Russian escalation or a wider war with NATO, or too likely to be captured by Moscow's advancing army. Others requested by Kyiv - including warplanes, battle tanks, and long-range precision missiles - continue to be denied as the Pentagon makes its own assessment of Ukrainian strategy and abilities.

There have been logistical problems, as supplies had to be located and donors cajoled and coordinated. Ukrainians, starting out with an arsenal stocked with aging, Soviet-era equipment, needed to be trained on modern Western armaments. Complicated transport routes to the war zone had to be arranged, along with provision of spare parts and repairs of the heavily used weapons.

But each phase of the conflict - Russia's initial, failed attempt to conquer Kyiv, the artillery battles in the eastern Donbas region, Ukraine's retaking of Kharkiv in the north and Kherson in the south - has brought more sophisticated armaments into the fight. What began as an ad hoc supply of small arms and short-range defenses today has become a torrent of precision systems with martial names and acronyms - Switchblades and HAWKS, HIMARS, NASAMS and now PATRIOTS.

Moscow's current attempts to extinguish the lights and turn off the heat across the country have led to new appeals for additional air defenses, something Ukraine has been asking for since Day One. Promised rush deliveries have begun to arrive, but not in the quantities Ukraine needs. "Is it enough?" Zelensky asked rhetorically amid his many expressions of gratitude during a Wednesday night address to Congress. "Honestly, not really."

But Ukraine's supporters are also seeking some basic changes in the way the war is being fought.

"We have to remember that this fight, this war, it is dynamic," Defense Secretary Lloyd Austin said in a recent interview in his Pentagon office. "When the situation on the battlefield changes, we have to be agile enough to change as well."

As combat has reached a stalemate with the arrival of winter, and while the still-outnumbered and outgunned Ukrainians prepare new offensives to regain more Russian-occupied territory, the plan is to train them to fight more like Americans.

Over the next several months, tens of thousands of Ukrainian troops - a massive increase from the relatively small groups that have been pulled off the battlefield to master new weapons this year - are being brought to Europe to learn and put to use new tactics against the deeply dug-in Russians in the eastern Donbas region and the southern Black Sea coast.

"I think if we can train larger formations - companies, battalions - on how to employ fires, create conditions for maneuver, and then be able to maneuver like you've seen [the U.S. military] maneuver on the battlefield, then I think we're in a different place. Then you don't need a million rounds" of artillery, a senior U.S. defense official said.

"We've got to get them to that point."

In late September, just days before Russia unilaterally declared its annexation of four Ukrainian regions in the eastern Donbas and the coastal south, Zelensky delivered a feisty address to students at Harvard University.

Moscow was still taking territory, and Ukrainian lives were being lost, while the West focused more on reacting to Russian actions than warding off new offensives, he said, looming down at his audience from a massive video screen.

"When you're preventing, you're taking the lead in the situation," Zelensky told the students. "Preventive action would mean that the world is not ready to swallow whatever the Russians want to feed it."

It was part grievance over the perceived stinginess of allies and partners by a president fighting for his country's survival, part theater by an actor-turned-politician who knows how to emote. The Biden administration was sympathetic but unmoved.

During a Wednesday news conference with Zelensky, President Biden said that provision of some of the weapons Ukraine wants could shatter unity among alliance partners who were "not looking to go to war with Russia."

As the West has become increasingly invested in a Ukrainian victory, benefactor and recipient have often been frustrated with each other. It began before the invasion, with what U.S. officials saw as the Zelensky government's refusal to take the threat seriously.

Preparations, and the early shipment of some defensive weapons, were made more difficult by the reluctance of Ukrainian military commanders to share their own plans with the Pentagon. Army Gen. Mark A. Milley, chairman of the Joint Chiefs of Staff, spoke regularly with his Ukrainian counterpart, Gen. Valery Zaluzhny, but did not get very far in the days and weeks leading up to the invasion.

"We were, like, 'What's your plan? Share your plan. Let us help,'" a second defense official recalled. "And the Ukrainians were 'We'll get it to you. Don't worry. We'll talk to you about it.'"

"In the end, they did not provide us with all that much information," the official added. "They were balancing their own internal politics. Or they had operational security reasons. Or whatever it was. But we did not have full visibility into there."

The Ukrainians, aware that the Americans had little faith in their ability to beat back the Russians, had frustrations of their own. To Zelensky's government, U.S. hesitancy made little sense if they were so certain the Russians were coming. Just days after Zelensky appointed him defense minister in November 2021, Aleksii Reznikov, a lawyer who had once served in the Soviet air force, met with Austin at the Pentagon to ask for Stingers, the portable, shoulder-launched air defense system that can shoot down helicopters and low-flying planes. "The answer was 'No, that's impossible,'" Reznikov said in a recent interview.

It was not until a few weeks before Russian troops crossed into Ukraine on Feb. 24 and moved to form a pincer around Kyiv that the impossible suddenly became possible. Stingers and U.S. Javelin antitank weapons, and their European equivalents, began to pour into the country, along with more ammunition for Ukraine's Soviet-era tanks and artillery.

Throughout March, Zelensky continued to call for Western aircraft and a no-fly zone over Ukrainian airspace, something the Americans and NATO found unthinkable for an alliance determined to keep the war at arm's length. Late that month, Reznikov flew to meet Austin in Warsaw with a new request.

"We had done our homework," Reznikov said. After studying publicly available information on the U.S. arsenal, the Ukrainians concluded that the Pentagon had a surplus of A-10s, the aging, subsonic attack aircraft known as Thunderbolts. "They can deliver heavier bombs, and we could use them against [Russian] tank columns," he said.

He asked for 100. Austin, he said, again replied it was "impossible" and "made no sense." The planes, Reznikov said he was told, were old-fashioned and slow, a "squeaky target" for Russia's formidable air defenses. "This was understandable to me. It was reasonable. I said okay," Reznikov recalled, throwing up his hands in mock surrender.

But the rejections, and explanations, continued.

Poland offered to send some of its Soviet-era MiG-29 fighter jets, but only if they were transferred to Ukraine via the U.S. air base in Germany. The Americans said they had no objections to the planes, but that transferring them from a U.S. installation sent the wrong message to Moscow.

Allies later collected spare parts to help get Ukraine's remaining MiGs into flying condition. The Americans also authorized the transfer to Ukraine a fleet of Russian-made, Mi-17 helicopters it had originally purchased for Afghan forces but never sent.

In April, Oksana Markarova, Ukraine's ambassador to the United States, visited General Atomics, the manufacturer of Gray Eagles, a next generation of lethal Predator drones. The company said it had aircraft available, but would need U.S. government approval to transfer them.

The administration denied the request, according to two U.S. officials familiar with the issue. Senior military officials expressed concern that if the drones were shot down or seized by Russian forces, the technology could be exploited by Moscow, which has turned to Iran to reinforce its own depleted unmanned aerial vehicle arsenal.

Despite the lack of sophisticated weaponry, and to the surprise of much of the world, Ukrainian forces outfought the sluggish Russians in the Kyiv region. By the end of March, Moscow's troops had withdrawn, and a new phase of the war that the United States, its allies, and Russian President Vladimir Putin had thought would be over in a few weeks was about to begin.

After Russian forces failed to take Kyiv, they regrouped to eastern Ukraine behind a wall of heavy artillery. They began to move west at a rapid pace, gobbling up territory far beyond the portion of the Donbas region Moscow had seized and illegally annexed in 2014. The Ukrainians were at a supply disadvantage, and senior U.S. military officials concluded they would lose without comparable or better equipment.

But decisions lagged on what to supply and who would supply it. The United States was providing the bulk of the weapons. Other countries in NATO and farther afield were also participating, but the process was haphazard and the hodgepodge of weapons was just barely holding Moscow's forces at bay. By mid-April, Austin saw the need to impose some order, and convened a meeting of international donors.

"We were pushing stuff into Ukraine at a very rapid pace early on, and encouraging other people to do the same," Austin later said in an interview in his Pentagon office. "But there really wasn't a unified, concerted effort to coordinate - not only the provision of materials, but also transport, deconfliction of routes" for the truck and train-loaded flow of weapons from European air and sea ports to the Polish border, where Ukrainian troops picked them up.

With less than a week's notice, top defense officials from more than 40 governments gathered on April 20 at the U.S. air base in Ramstein, Germany, to establish a system to find the right weapons, make decisions and coordinate deliveries. "We've got to move at the speed of war," Austin told the closed-door session. Suddenly, U.S. reluctance to provide American-made, high-powered artillery evaporated and Biden approved the first shipment of powerful howitzers, the M777 155 mm systems, to Ukraine.

It was, Reznikov said, "the first step across the Rubicon."

But agreement didn't necessarily mean timely arrival. The first U.S. howitzers did not ship to Ukraine until June. And in Ukraine's view, there has never been enough artillery to match Russia's overwhelming advantage.

Shells for Ukraine's old Soviet systems are no longer produced in mass quantities, and acquiring them abroad is unpredictable. Arms brokers representing Ukraine and Russia compete to outbid each other for the little remaining stock.

A reporter traveling near the front lines outside of Kherson this fall found one Ukrainian soldier nervously awaiting delivery of more ammunition for a Soviet-era artillery piece that was older than he was. The 40 or so shells rolling around in the back of a truck were all he had left, said the 25-year-old platoon commander in Ukraine's 59th Motorized Brigade who asked to be identified by his call sign, Vognyk. Some soldiers in his unit had driven north to pick up some more they were told had been left behind by retreating Russian soldiers near Kharkiv.

"There is always a shortage," Vognyk said. "We just have to wait for a good target before we use anything." He was still awaiting his turn for training on the M777.

The next step toward providing more sophisticated weaponry came in early summer, as Ukraine sought artillery that would fire with more precision, and deeper behind Russian front lines. But when Reznikov asked for High Mobility Artillery Rocket Systems, the multiple-launch HIMARS, the Americans drew another line.

HIMARS were in relatively short supply in U.S. stockpiles and had been transferred only to a handful of countries, including Jordan, Qatar and the United Arab Emirates in the Middle East, Romania and Poland in Europe and, in 2020, to Taiwan. There were implications for U.S. force readiness and legal concerns about the transfer of sensitive technology. How would the Ukrainians be trained to use them, and how would they be maintained in the field?

"It's not just about what you have" on the battlefield, Austin said in the interview. "It's about how you use what you have . . . and whether or not you train on how to use it." The constant need for maintenance, spare parts and fuel gets "kind of lost on a lot of people. . . . If you can't sustain a system in a fight, you might as well not even bother to deploy it."

It was not until June, just as the first M777s were arriving, that Biden announced approval of four HIMARS systems, and an unspecified amount of Guided Multiple Launch Rocket systems munitions, or GMLRS, the medium-range missiles to be fired from them. Agreement came only after the top levels of the Ukrainian government assured the White House they would not be used to fire into Russian territory. Biden drew another line at ATACMS, missiles with a range of up to 200 miles.

Another step across the Rubicon had been taken, Reznikov said. So then, "I asked for tanks." Once again, the United States said no, insisting that the M1 Abrams tank was too sophisticated for Ukrainian troops and required too much logistical support to operate. Germany has also refused to supply its Leopard and Marder tanks, in large part, German officials have said, because they don't want to be the first to contribute a major new weapon system that the United States has not yet agreed to.

Reznikov remains optimistic. Tanks, he said, are "low-hanging fruit," and German minds would change if the United States "like a big brother," would supply the M1 Abrams. "One. Just a symbolic step, and after that I'm sure we will have Leopards from Germany, we will have British and French tanks."

"I've got a lot examples with Stingers, artillery, HIMARS and more" about how the U.S. approval process works, he said. "It's just a political decision. I absolutely understand that all the pro-Ukraine politicians in different countries have to have an internal agenda. It's normal," he said.

Told of Reznikov's comments, the first senior defense official said even one M1 was out of the question. Used by U.S. forces in Iraq, the massive battle tanks were "hard for us to sustain and maintain. It would be impossible for them." Besides, "I think that the Ukrainians now have more tanks on the battlefield than the Russians do. . . . The Russians have graciously donated a lot" the official said sarcastically, noting the equipment that Moscow's forces abandoned in hasty retreats from Kyiv and Kharkiv.

The industrial challenge of a conventional war

With the volume of aid have come questions about how long such effort and expense can be sustained in a time of global economic pain. Biden has committed more than $20 billion worth of weapons into Ukraine, $14 billion of it in drawdowns from Pentagon weapons stocks, and $6 billion in new weapons production contracts.

"There's the surge" of weapons most of this year, "and then there's what's sustainable," Colin Kahl, the Pentagon's policy chief, told a defense forum in September. "We don't want to be in a position where we surged and then kind of fall off to nothing. . . . There is no expectation that it's going to be what we've seen in the last six months . . . but it's not going to be zero."

A new Republican House majority is already making noise that the United States will not continue to write a "blank check" to Ukraine. While politicians argue about the expense, the Pentagon is increasingly concerned about supply - both for Ukrainian forces and for American readiness to fight other potential battles.

Years after U.S. defense officials shifted their focus from more conventional warfare to counterterrorism and space-aged weaponry, Ukraine has shown that trench battles in Europe aren't confined to the history books. A nonnuclear war with China, or even directly with Russia, is likely to require a steady, long-term stream of the kind of equipment that is now in short supply.

"A conventional war . . . is an industrial war," said Seth Jones, a former adviser to U.S. Special Forces who now heads the International Security Program at the Washington-based Center for Strategic and International Studies (CSIS). There are "serious challenges" to current supplies in the U.S. arsenal, he said. "We are really low . . . and we're not even fighting."

An upcoming CSIS report on American readiness, Jones said, concludes that "the U.S. defense industrial base is in pretty poor shape right now. If you identify China as the 'pacing' threat, and an 'acute' threat from Russia, we don't make it past four or five days in a war game before we run out of precision missiles."

The United States has provided Ukraine with air defense systems, from Stingers taken out of U.S. storage, to NASAMS, the medium range National Advanced Surface-to-Air Missile System jointly produced by Raytheon and Norway's Konsberg company, and has promised far more. But "unfortunately," according to Mark F. Cancian, a retired Marine colonel and senior adviser at CSIS, "turning good intentions into battlefield realities will be difficult."

Stinger production lines had long since been closed down. The first two of eight promised NASAMS have recently arrived in Ukraine, but the other six have yet to be manufactured and delivery could take up to two years.

"This is a very real challenge," said Norway's deputy foreign minister, Eivind Vad Petersson, in an interview. "I've been surprised by my own impatience with the defense industry because it's such a different animal to all other kinds of fields. . . . Just imagine any other business sector, looking at this kind of scenario where stocks are empty and need to be refilled, and there's a clear political will and need to do this to support Ukraine."

"Industry would rush to ramp up production because there's obviously going to be demand," he said. "But the defense industry doesn't work that way."

Though the Ukraine war has been a boon to defense spending, production is suffering the same problems as other industries - inflation, supply chain shortages, a dearth of skilled and willing workers, and general post-pandemic lags. But the unique peculiarities of its contracting systems, requiring long lead times and prepayment; the tendency of defense budgeters during peacetime to save money by cutting back on more prosaic items such as precision munitions in favor of ships, planes and other big-ticket items that please lawmakers; and the immediate and unanticipated demands of the Ukraine war have all played a part.

A shortage of artillery ammunition of all sorts remains a weakness. Although production increases are planned, the U.S. defense industry can presently build about 14,000 155 mm howitzer rounds per month, Army Secretary Christine Wormuth said in an interview. According to U.S. defense officials, Ukrainian forces have fired that amount in two days during periods of heavy fighting.

One partial solution, discussed by Austin at the November international contact group meeting and at high level NATO gatherings, is encouraging the Europeans to more fully open their own stockpiles and build up their own industrial capacity to take more of the burden. Spain has provided four HAWK medium-range air defense systems for Ukraine, and the United States, which retired its own use of the system in 2002, is sending munitions for them.

Germany has promised to supply Ukraine with four IRIS-T systems, a relatively new ground-based version that uses its infrared air-to-air IRIS missile. But only one has been sent so far; the others are still on production lines.

While the Western weapons pipeline to Ukraine remains open, the back and forth over the next best system - and the urgencies of the fluid fronts of the war - remain in constant negotiation.

"Clearly Mr. Zelensky has made plain and apparent . . . his desire for additional military capabilities, and who can blame him given the aggression inside his country," John Kirby, communications coordinator for the National Security Council, said this week. "Any president, any commander in chief in similar circumstances, would want as much as he can get as fast as he can get."

"We also have no interest in escalating this war in a manner than makes it the United States versus Russia," Kirby said. "But nobody gets a veto over what the United States provides Ukraine."

Khurshudyan reported from Kyiv.