Just weeks after Russia invaded Ukraine in February 2022, Ukrainian troops began running out of ammunition for their aging Soviet-era artillery.

The United States was soon scouring foreign arsenals around the globe for the right kinds of shells as part of its pledge to support Kyiv against its much better equipped adversary. But the Pentagon knew it would never be able to get enough, as fewer nations made Russian heavy artillery ammunition and many Cold War stockpiles of them had become unusable with age.

So on a Saturday that spring, Defense Secretary Lloyd J. Austin III called Gen. Mark A. Milley, his chairman of the Joint Chiefs of Staff at the time, to figure out how many American howitzers could be quickly shipped to Ukraine along with newly made high-explosive shells.

That call set into motion a chain of events that led to a pipeline of arms for Ukraine, and a reshaping of how the United States envisions building alliances as it pushes back on Russia’s influence.

Officials in the Biden administration turned first to U.S. allies for help. But they also drew on relationships developed over years with the militaries of non-NATO countries to build a network to aid Ukraine, something the administration considers a shining example of how its focus on strengthening alliances has paid dividends to U.S. interests around the world.

On Thursday, the collective of nations, known as the Ukraine Defense Contact Group, is gathering at Ramstein Air Base in Germany for the 25th and final time under the Biden administration.

Whether it continues under the incoming administration’s leadership is uncertain. President-elect Donald J. Trump is deeply skeptical of supporting Ukraine, puts much less stock in alliances and has openly curried favor with President Vladimir V. Putin of Russia.

When the contact group first met on April 26, 2022, just 61 days after Russia invaded, the United States, Britain, France and a handful of other nations had been supporting Kyiv individually. But dozens more joined them in Germany to hear a battlefield update directly from their Ukrainian counterparts.

The massacre of civilians in Bucha, a Kyiv suburb, had recently come to light. Mr. Austin and General Milley made it clear that similar atrocities were likely to happen without more arms and ammunition for Ukraine, and asked the assembled dignitaries to dig deep into their arsenals for desperately needed supplies.

The meeting was put together quickly, on just four days’ notice at Ramstein, chosen for its central location in Europe, its long runway for delegations arriving by plane and the ability to maintain tight security while hosting sensitive discussions about Ukraine’s future.

In a nondescript ballroom at the base’s officers’ club, Mr. Austin and General Milley, flanked by their Ukrainian and German counterparts, sat at the head of a horseshoe arrangement of folding tables alongside representatives from 40 countries.

It has since grown to at least 52 countries, and the Pentagon has hinted that others have kept their participation secret.

They have met roughly every month since.

A global battle for arms and influence

Shortly after Mr. Austin landed at Ramstein in a C-17 cargo plane on Wednesday morning, snow began falling on hulking Air Force transport jets along the runway. He prepared for the next day’s meeting in a nearby hotel on base.

It would be the final contact group meeting of his tenure as defense secretary.

That evening, the Ukrainian defense minister, Rustem Umerov, and two aides walked the halls of the hotel’s second floor in military fatigues to join a private meeting with Mr. Austin.

The Ukrainians traveled light, without the sort of large security detail that hovered nearby for his American counterpart.

The troops Mr. Umerov leads in some ways still straddle the two defining military cultures of the Cold War — that of the U.S. and NATO, and that of the former Soviet Union.

Over decades, the two sides created their own ecosystems of arms that were compatible with those of allies, but not with those of their foes. And while the differences between the 152-millimeter shells fired by Russian-made artillery pieces and the 155-millimeter version adopted by NATO may seem small, they are emblematic of how militaries around the world have long been divided into one of two camps.

Determining whether a country was aligned with the West or with Moscow was often as easy as spotting which weapons it used.

But the number of countries manufacturing Russian-designed weapons has dwindled, especially as many former Soviet republics that once produced them have since joined NATO.

Global supplies of those arms have been further reduced as Russia has paused much of its arms exports to maintain sufficient ammunition supplies for its own needs in Ukraine.

The change has left many of Russia’s traditional clients looking elsewhere for ordnance just as more countries around the world have begun producing NATO-standard weapons — even if they are not formal members of the alliance.

A new type of alliance is born

The conversion of Ukraine — a former member of the Soviet Union — over to NATO warplanes like F-16s and arms like HIMARS mobile rocket launchers has pulled even more countries away from Russia’s orbit.

That shift is evident in the makeup of the contact group itself, which contains 20 nations formerly in Moscow’s Cold War sphere of influence — some of which continued to purchase Russian arms until the invasion of Ukraine.

The contact group’s composition shows a novel approach for American power projection — one that U.S. officials have said future administrations could use in case of major conflicts, such as a potential Chinese attack on Taiwan.

While NATO has always been an alliance of nations who have pledged to come to each others’ aid if attacked, its influence has expanded far beyond its formal membership of 32 nations through partnerships with dozens of other countries since the mid-1990s. The United States leveraged that network in creating the contact group.

In addition to the NATO member states, many of the countries that participate in the contact group are what the State Department calls “major non-NATO allies.” They include Argentina, Australia, Colombia, Israel, Japan, Kenya, Morocco, New Zealand, Qatar, South Korea and Tunisia.

After decades of arms sales, these countries collectively had a deep inventory of NATO-standard arms to share with Ukraine.

Others are on NATO’s periphery, with Bosnia, Georgia, Ireland, Kosovo and Moldova joining the group as well as Ecuador and Peru, two former Russian military clients that have entered into partnerships with the alliance.

Lessons from the battle against ISIS

Mr. Austin modeled the contact group on the Coalition to Defeat the Islamic State, which formed in September 2014 and came to encompass more than 80 nations.

At that time, Mr. Austin was an Army general in charge of U.S. forces in the Middle East. Eight years later in Ramstein, he shook the hands of many of the same leaders who had supported him in uniform.

Together, the nations of the group supporting Ukraine have supplied Kyiv with more than $126 billion in military aid, funding and hardware, according to the Pentagon.

While the defense secretary worked on growing and organizing the new coalition, the State Department worked behind the scenes to get even more of Russia’s former military client states to donate their Soviet-era arms to Kyiv in exchange for financial grants and expedited access to the latest American weaponry.

Two and a half years into the war, a Defense Department office still updates a file every week called The Matrix — a spreadsheet of the countries known to have Russian weapons along with their likely inventories.

It also includes what the United States is prepared to ask them for on Ukraine’s behalf, and a list of incentives Washington can offer in exchange thanks to an influx of money authorized by Congress.

Whether such efforts continue under the next administration is unknown, but it is clear that military and civilian officials have considered the possibility that Ukraine may have to fight on without its single biggest benefactor.

Should the Trump administration decide to leave the Ukraine Defense Contact Group, Pentagon officials have said, another country could take up the U.S. leadership role, continuing the global effort to supply arms to Ukraine without Washington’s influence.