This is part three of a series examining the challenges confronting the NATO alliance.

LVIV, Ukraine — Four years into Russia’s invasion of Ukraine, officials across NATO’s eastern flank increasingly believe the alliance’s future is already being rewritten on Ukraine’s battlefield.

From drone warfare and cyber defense to civilian resilience and large-scale military mobilization, Eastern European officials say Ukraine has become one of the world’s most battle-tested militaries, forcing NATO to rethink how future wars will be fought.

This week, NATO Secretary General Mark Rutte confirmed Ukrainian President Volodymyr Zelenskyy had been invited to attend the alliance’s annual summit in Ankara in July, underscoring how central Ukraine has become to NATO’s future despite not being a member of the alliance.

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"I think today the Ukrainian army is the number one army in Europe," Mayor of Lviv Andriy Sadovyi told Fox News Digital during an interview in the western Ukrainian city.

"I think NATO needs the Ukrainian army," he added.

The debate over NATO’s future intensified this week as alliance foreign ministers gathered in Sweden ahead of a major NATO summit in July, with Secretary of State Marco Rubio calling the upcoming meeting "one of the more important leaders’ summits in the history of NATO."

Rubio warned NATO allies this week that the alliance lacks sufficient munitions production for future conflicts, a concern echoed by Ret. Lt. Gen. Richard Newton, who said the Pentagon is studying Ukraine’s rapid wartime industrial adaptation.

"A number of nations are taking a page out of Ukraine’s transformation of its defense industrial base, in terms of quality as well as the tremendous increase in quantity of arms to the frontlines as well," Newton said, adding, "The Pentagon is taking note and working to encourage the transformation of our own industrial base so we can drastically improve and more rapidly provide capabilities to our forces in the field, not in a matter of years but in months and perhaps even in weeks."

Rubio also referenced President Donald Trump’s announcement that the U.S. would maintain troop deployments in Poland after concerns earlier this week about possible reductions on NATO’s eastern flank.

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Speaking before the NATO meeting, Polish Foreign Minister Radosław Sikorski welcomed Trump’s announcement. "I want to thank President Trump for his announcement that the presence of American troops in Poland will be maintained more or less at previous levels," Sikorski said.

"I think this makes Putin very uncomfortable."

Some note that the debate over NATO’s future comes with deep irony for Moscow.

One of Russian President Vladimir Putin’s central grievances before the invasion was NATO’s eastward expansion and Ukraine’s growing ambitions to move closer to the alliance. Moscow repeatedly demanded NATO roll back its military footprint to pre-1997 levels and opposed any future Ukrainian membership.

Instead, the invasion accelerated NATO’s expansion.

Finland formally joined NATO in 2023, ending decades of military nonalignment, while Sweden joined in 2024 after Russia’s invasion dramatically reshaped security calculations across northern Europe. Finland alone added more than 800 miles of direct NATO border with Russia.

Now officials in Poland and Ukraine say the war is not only expanding NATO geographically, but fundamentally transforming the alliance itself.

"For decades, NATO focused largely on expeditionary wars and counterterrorism," said Polish Deputy Defense Minister Paweł Zalewski during an interview in Warsaw. "Modern warfare is mostly done by drones."

"There is not a military in the world which is better than Ukraine" in understanding today’s battlefield realities, he added.

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Ret. Gen. Philip Breedlove, who served as NATO Supreme Allied Commander Europe, said the war has fundamentally transformed how militaries around the world understand modern warfare.

"The war in Ukraine has changed far more than just NATO’s understanding of modern warfare — it has changed the whole world’s understanding," Breedlove told Fox News Digital.

Breedlove added that Ukraine’s military has evolved into "one of Europe’s most capable and formidable" forces after years of fighting Russia, despite having surrendered its Soviet-era nuclear arsenal under the 1994 Budapest Memorandum.

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"Today, most agree that Ukraine is not only fighting, but winning back land against one of the world’s most formidable forces," he said.

That transformation is visible throughout Ukraine.

Before Russia’s invasion, Ukraine had one of Eastern Europe’s largest IT sectors. Sadovyi said the war forced much of that technological ecosystem to pivot toward defense production.

"Before the invasion, we had in Kyiv a huge IT cluster, 40,000 workers," Lviv’s mayor said. "During the war, we rebuilt the IT cluster to defend cluster."

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Ukraine now operates a rapidly expanding wartime innovation ecosystem focused on drones, anti-drone systems, battlefield communications and decentralized weapons production. NATO officials and European militaries are increasingly studying those lessons closely.

Breedlove says the conflict exposed the limits of traditional air power and accelerated the rise of drone warfare.

"It’s critical to remember that the war in Ukraine is being fundamentally fought without the support of modern air warfare because of the failures of the Russian Air Force," he said.

"It’s why drone warfare has grown so exponentially, because neither side was able to marshal true modern air capabilities."

The changes are also reshaping NATO strategy.

The Polish defense official Zalewski told Fox News Digital the Pentagon is now promoting what Polish officials describe as "NATO 3.0," a model in which Europe assumes greater responsibility for conventional defense as the United States shifts more attention toward China and the Indo-Pacific.

"The main assumption of this concept is that conventionally it would be Europe defending itself," he said.

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That shift comes as Poland dramatically increases military spending and positions itself as one of NATO’s leading military powers on the alliance’s eastern flank. Warsaw spent nearly 5% of GDP on defense this year, the highest level in NATO.

Polish officials argue the war proved Eastern Europe was right to take Russia’s threat seriously long before many Western European countries did.

"The eastern flank is much more powerful than even five years ago," Polish Deputy Foreign Minister Marcin Bosacki told Fox News Digital during reporting in Warsaw.

"We were right about the nature of Putin’s regime and Russia’s aggressive strategy."

Ukraine is not currently a NATO member, and the alliance has avoided offering Kyiv a concrete timeline for accession during the war out of concern it could trigger direct confrontation between NATO and Russia.

But across Eastern Europe, officials increasingly argue the alliance’s future may depend on Ukraine regardless of formal membership.