IN POLAND, NEAR THE UKRAINIAN BORDER — Just off a runway on a Polish airfield, forklifts busily emptied an Air Force C-17 transport jet of its cargo alongside a much smaller civilian propeller-driven plane, ferrying pallets of green boxes full of munitions from each to a nearby asphalt parking lot filling up with many dozens of them.
Some bore American-made weapons, while others held a variety of ordnance made in Eastern Europe — all of them representative of Ukraine’s highest priorities for military aid that would soon be loaded into a fleet of waiting tractor-trailer trucks loitering nearby for the journey into Ukraine.
The Pentagon sources much of the American-made weaponry it sends to Kyiv from its own stockpiles, but relies on American defense contractors to scour Eastern European munitions factories to find newly made weapons designed by the United States’ former adversary, the Soviet Union, to fulfill President Biden’s pledges of increased military aid for Ukraine.
Ukraine still uses many weapons common to the Russian army, such as modern Kalashnikovs. And while Ukraine’s pleas for more sophisticated weaponry — such as Javelin anti-tank and Stinger antiaircraft missiles — have received widespread attention, the country’s military has pressing needs for a wide range of munitions, including tens of millions of rounds for Soviet-era arms that are not on the cutting edge but are staples of the Ukrainian military.
The Pentagon calls such arms, including rockets, artillery shells and ammunition for machine guns and assault rifles, “nonstandard ammunition” — given that the munitions are incompatible with those used by the United States and many allied nations, which are generally known as NATO-standard ammunition.
And since the Sept. 11 attacks, the Pentagon has been buying large amounts of such weapons through a variety of American defense firms to supply client armies in Afghanistan, Iraq, Syria and other countries that still rely on Soviet-designed arms.
One of those companies is the Ultra Defense Corp. in Tampa, Fla., which has about 60 employees and has built a bustling business working with factories in Romania, Bosnia, Serbia, Slovakia, Czech Republic and Bulgaria.
Those countries provide about 90 percent of the nonstandard ammunition purchased by the Pentagon, according to Matthew Herring, the company’s owner, though his firm provides just a fraction of the Pentagon’s total orders.
Mr. Herring, who bought the company in 2011 when it was a three-person firm providing Russian-made helicopters to Afghan forces, is now in Poland meeting with Ukrainian officials to find out what else his company can do to provide them with Eastern Bloc munitions.
“A month ago, when Kyiv was surrounded, it was, ‘What do we need in the next 48 hours?’” Mr. Herring said. “But now the Ukrainians are digging in for a long fight and it’s, ‘How do we get enough to sustain us in this fight?’”
“So it’s a longer view about what they now need,” he added.
The Pentagon’s nonstandard ammunition program was built in direct response to an investigation by The New York Times in 2008 that exposed illegal sales of Chinese-made arms to the U.S. Army in Afghanistan, which became the subject of the 2016 movie “War Dogs.”
According to Mr. Herring, after that scandal, the Pentagon contracted with large defense firms to provide nonstandard ammunition for Afghanistan and later allowed small companies like his to offer bids for the same kinds of services.
Whether certain European nations that still make Soviet-designed munitions will sell their wares to Ukraine is a political decision — one that may depend in part on whether they value maintaining a good relationship with President Vladimir V. Putin of Russia.
Representative Jason Crow, Democrat of Colorado, a former Army Ranger who serves on the House Armed Services and Intelligence Committees, said in an interview last week that much of Ukraine’s nonstandard ammunition “very rapidly will be depleted” because of the current pace of combat with Russia.
The Ukrainian military will ultimately need to transition to NATO-standard weapons in the future, he said, so that it can further take advantage of the West’s vast stockpiles of ammunition sitting in bunkers across Europe and the United States.
That move is already underway, in part, through the Pentagon’s provision of five battalions’ worth of 155-mm howitzers to fulfill Ukraine’s pressing needs for what it calls long-range fires, which are similar in capability to the Soviet-designed 152-millimeter guns that Ukraine has been using against Russia.
So while businesses like the Ultra Defense Corp. will still buy as many 152-millimeter artillery shells as it can for Ukraine’s legacy artillery weapons, the Pentagon is aggressively moving in 184,000 shells from its stockpile in Europe for the 155-millimeter howitzers it has pulled from Army and Marine Corps stockpiles in the United States and shipped to Kyiv.
At a news briefing last week, John F. Kirby, the Pentagon press secretary, said nonstandard ammunition remained an important part of the supply of arms the United States is providing Ukraine.
“It’s the lifeblood here for the Ukrainian armed forces,” Mr. Kirby said of the ammunition supplies being given to Kyiv. “We don’t talk a lot about small arms ammunition. It doesn’t get the headlines, I understand that, but at every discussion we have with the Ukrainians, they talk about how important that is.”
Since the invasion, he said, the United States has coordinated and delivered more than 50 million rounds of small arms ammunition to Ukraine, much of it Soviet-designed. Mr. Kirby said the United States was continuing to “talk to allies and partners about their inventories of nonstandard ammunition” in an effort to get more munitions to Ukraine.
“It is having a truly significant impact on the battlefield,” he said of the Soviet-designed ordnance. “They use that ammunition literally every day in defending their country.”
John Ismay reported from an undisclosed location in Poland near the Ukrainian border and Eric Schmitt from Washington.