ON THE POLAND-UKRAINE BORDER — There were no passport officers on the dirt road, no customs lane, no signs marking this isolated patch of farmland for what it has become: a clandestine gateway for military supplies entering Ukraine.

“No pictures, no pictures,” shouted a Polish border guard as a convoy of 17 trucks hissed to a halt on a biting morning earlier this week.

Not far from here was a Ukrainian military base where at least 35 people had been killed a few days earlier by a Russian missile barrage, and no one wanted to call attention to this ad hoc border crossing. Washington Post journalists were directed to turn off the geolocation function of their cameras.

The convoy was carrying 45 vehicles — retrofitted Jeeps, ambulances, an armored bank truck and an army field kitchen — as well as 24 tons of diesel. It had traveled overnight from Lithuania as part of a swelling supply network racing to catch up with the return of war to Europe. More than a dozen volunteer drivers, including one whose relief work was normally limited to helping motorists stranded on the highway, had driven hood-to-taillight almost around-the-clock to rendezvous with Ukrainian fighters.

While governments negotiate over fighter jets and high-end weapon systems, soldiers on the ground are struggling to fill more basic needs. With Ukraine’s own factories shut down by shelling, its forces rely increasingly on volunteer, pop-up supply chains like this one for vital gear, including body armor, medical supplies and the pickup trucks and SUVs they covet as fighting vehicles.

A second convoy was scheduled to arrive later in the day, packed with generators, radios, surveillance drones, night-vision gear and, most coveted of all, almost 7,000 bulletproof vests and helmets. For the soldiers, it is a lifeline.

“That is what we need the most,” said Lt. Andrey Bystriyk, one of the many Ukrainian fighters who had traveled across his war-ravaged country to meet the convoys. His blue eyes teared up when he talked about the aid pouring in from neighboring countries.

“From the army, we get the gun and the ammunition and the uniform,” he said. “But under the uniform, what we eat, what keeps us safe, how we move around and fight — that comes from the people, our people and foreign people.”

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The journey began hundreds of miles to the north in a warehouse in Lithuania, a country not usually thought of as a military supply hub.

But this tiny Baltic nation has seen a huge outpouring of support for Ukraine, imagining what Russian President Vladimir Putin might have in store for it should he prevail in his current invasion. Vilnius, Lithuania’s small medieval-era capital, is filled with blue and yellow Ukrainian flags.

Much of the donated money and supplies has flowed to Blue and Yellow, a nonprofit founded in 2014 to supply Ukrainians fighting the takeover of eastern Ukraine by Russian-backed separatists. Now the group is the focal point of a country’s yearning to help.

“It has just exploded,” said Jonas Ohman, a Swedish-born filmmaker who started the group.

For years, Ohman said, he took no salary and had no paid staff as he fulfilled direct requests from front-line units with an annual budget of less than $200,000. Since the invasion last month, more than $20 million has poured in from within Lithuania, a country of 2.8 million residents. He is dispatching a convoy to the border every four or five days.

With a cellphone held against a days-old beard, Ohman orders military gear by the ton from around Europe, China, Israel. He argues with customs officials in a half-dozen countries to get the shipments delivered, railing against functionaries who block his way and officers who are slaves to regulation.

“I tell them all the time: 10,000 euros can be more deadly than a million if you know how to spend it,” he growled between phone calls.

Ohman has filled one donated warehouse on the outskirts of Poland’s capital, Warsaw. Another in Vilnius, provided by a Lithuanian transport company, has become a drop-off site for locals wanting to give.

“These will work,” one volunteer declared on a recent afternoon when a truck arrived at the Vilnius warehouse with 800 pairs of new steel-toed boots and 1,000 fleece jackets still in the wrapper, all donated by a hunting goods retailer.

A forklift unloaded the cases, depositing them next to 14 pallets of IV saline solutions and boxes filled with 13,000 trauma tourniquets and 200 satellite phones.

A local marketing company has launched a fundraising campaign for the group. And a group of Rotary Club volunteers makes calls to military suppliers in surrounding countries.

“Everything in Europe is selling out,” said Zemyna Bliumenzonaite, a Blue and Yellow staffer. “But we are getting more requests than ever.”

She held out her phone to show some of the texts she gets from soldiers in Ukraine. One named “Kruk” asked for 1,000 tourniquets and 40 individual first aid kits. She tells him they will be in the next convoy.

“You are our Guardian Angel,” he writes back.

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“I heard they needed bigger vehicles and four-wheel drives,” said Dainius Navikas, 43, a Vilnius management consultant who immediately thought of his black 2015 Grand Cherokee. “I had no choice. The Ukrainians are fighting for us.”

Navikas and wife drove the Jeep — along with an extra set of winter tires — to a designated garage on the outskirts of the Lithuanian capital. They found a lot packed with dozens of vehicles ready to be processed and shipped to Ukraine.

Some had been signed over by their owners. Others had been bought by Blue and Yellow.

“When they hear we are buying for Ukraine, a lot them of them drop the price immediately,” said Lukas Pacevicius, the owner of the garage, who has largely suspended his regular business activities.

Working overnights and weekends, mechanics check the engines, sending them out to transmission or brake shops if needed. Armor plating is welded to some of the pickups, following specifications provided by the soldiers.

On a recent day, dozens of volunteers were scrambling around the vehicles, covering their windows and headlights with paper and masking tape ahead of repainting the bodies. Workers dodged the vehicles as they were shuttled from one part of the line to another.

Two men wearing Tyvek suits and respirators, well practiced in painting and not too fastidious, transformed Navikas’s glossy black Grand Cherokee into a dull green patrol vehicle in under 20 minutes. And then a Mercedes Sprinter, and then a Nissan Pathfinder. An olive mist hung over the entire workshop.

“We want to cover every reflective surface, even the bumpers and wheels,” said Rolandas Jundo, the owner of a sign company who was applying window tinting to a Land Rover that still reeked of paint.

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Three days later, gassed up with donated fuel, most of the vehicles were driven onto car carriers. Two local tow trucks hitched up four more vehicles. Four men wrangled a military mobile kitchen into a panel truck.

With the sun still high, the convoy pulled out, flanked by a pair of Lithuanian police cars. Just outside of Vilnius, a group of people on a pedestrian bridge shouted and pumped their fists when the odd parade rolled under.

“It feels very important,” said one of the drivers, who like several volunteers spoke on the condition of anonymity due to a combination of modesty and security concerns. “We still have a lot of crazy Fifth-Column types around,” said another driver, referring to Russian sympathizers.

The convoy moved as fast as its slowest truck, about 50 miles an hour on average. At a gas station just before the Polish border, Lithuanian police handed off to their Polish counterparts. Sometime after 2 a.m., everyone pulled into a rest area north of Warsaw for two hours of sleep.

By dawn, forests had given way to rolling fields. The police escort kept their lights flashing and sounded their sirens as the trucks rumbled through red lights. Surprised locals stared from village sidewalks.

Nineteen hours and many cans of Red Bull later, the convoy pulled up at the unmarked entrance to Ukraine.

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Bystriyk, an officer with the Zaporizhzhia Territorial Defense Brigade, had just endured his own all-night drive to reach the rendezvous. His was one of about 20 Ukrainian units, both regular military and volunteer militia, that had dispatched representatives to meet the convoy.

Bystriyk had driven about 11 hours from the area around the besieged city of Dnipro in eastern Ukraine in hopes of getting vehicles and an upgrade on the body armor that most of his men now wear: homemade vests cobbled together by local residents with steel and canvas. “They try to bend it like a body shape, but it doesn’t work,” he said.

It would take about 3,000 sets of body armor to fully outfit his men, Bystriyk said. He had been told he might get as many as 400 when the second expected convoy arrived. In the meantime, he eagerly eyed the vehicles that were carried by the first one.

“Stingers and Javelins are critical of course,” he said of the antiaircraft and antitank missiles. “But for us, these vehicles are essential. They are our firepower, our mobility.”

Ukrainian soldiers drove them to a spot where border officials would fill out paperwork and then the vehicles would be distributed. One soldier made a beeline for a brand new CForce quad ATV — to be used in cavalry-like raids by Ukrainian Special Forces — and rode off with a grin.

Bystriyk looked for a truck that his men could mount with a rocket launcher or machine gun, creating one of the “specials” common with fighters in Libya, Syria and other recent hot spots. There weren’t as many pickups as in a delivery a week earlier, but he was glad to see Pathfinders, Freelanders, Pajeros.

Videos posted by Ukrainian fighters on social media show teams in SUVs like these outmaneuvering Russian armored vehicles, popping out from forests or side streets to hit them with rocket-propelled grenades and dashing away.

“Every day the Russians try to enter Zaporizhzhia and every day we have stopped them,” Bystriyk. “We need these cars. And we are thankful the Lithuanians are bringing them.”

In the end, Bystriyk was satisfied with a beefy Nissan Patrol to drive back to the war. But he learned that the convoy with the vests and helmets would be delayed because of a customs hang-up.

He would be back at this unlikely supply site, he knew. Probably many times.

“We need a lot,” he said. “And the need is still growing.”