This spring, a Ukrainian defense ministry employee was reviewing Denmark’s military aid commitments when she found a problem buried in the fine print. Thousands of artillery shells earmarked for Ukraine were the wrong kind — short-range ammunition that would not enable guns to strike deep behind Russian lines.

Skipping the usual bureaucratic process, the staffer worked the phones around the clock, pushing Danish officials to modify the aid package, according to her boss, Oleksii Antoniuk, deputy head of the defense ministry’s cooperation department. Within weeks, she had secured 15,000 long-range shells.

“If not for her, this wouldn’t have happened,” Mr. Antoniuk said. “These shells really could mean a thousand Russians dead. That’s literally her contribution to the war effort.”

The staffer, whose identity cannot be disclosed for security reasons, is not a veteran military procurement specialist. She’s in her early 20s, fresh out of university.

Mr. Antoniuk is barely older, just 24.

They embody a generational shift sweeping through Ukraine’s defense sector, where young men and women, many of them under 30, are gradually displacing a Soviet-era old guard and establishing themselves as a driving force behind their country’s war effort.

The shift runs through every link in Ukraine’s war machine. Twentysomething engineers design sophisticated drones. Young entrepreneurs turn those prototypes into production lines, some using their Western educations to attract foreign funding. At the defense ministry, recent graduates cut red tape to speed weapons to the front.

On Thursday, those same young people took to the streets to protest the ouster of Ukraine’s defense minister, Mykhailo Fedorov, laying bare the generational divide running through the country’s war effort. Mr. Fedorov, 35, had pushed to reform weapons procurement and wage a tech-driven war, winning strong support among young Ukrainians but angering part of the military and defense industry establishment that pressed for his dismissal.

The rise of a younger generation has been driven in part by military necessity. Unable to match Russia in manpower and firepower, Ukraine has bet on out-innovating its larger adversary. That has opened the door to a new generation steeped in start-up culture and new technologies — a change nowhere more evident than in Kyiv’s embrace of drone warfare.

“This level of innovation, and the way it’s been integrated into Ukraine’s fight, wouldn’t be possible without these young people,” said Per Holst, the deputy defense attaché at the Danish Embassy in Kyiv, who oversaw the delivery of long-range artillery shells.

Over time, defense has eclipsed I.T. as the biggest draw for Ukraine’s young talents. Part of the appeal is how much the two fields overlap, with similarly rapid changes. Another is that defense work offers a role in the national fight for survival, but without the dangers of military service, and some of the jobs exempt men from conscription.

“Humanity will survive without apps,” said Andriy Zagorodnyuk, a former Ukrainian defense minister and now a security analyst. “But by working for defense, you’re saving your country. It’s a totally different meaning.”

Mykhailo Rudominski’s path into defense began with a call from a friend, a soldier helping defend Kyiv from Russia’s invasion in 2022.

Ukrainian troops had a problem, his friend said. Lacking expensive military radios, they relied on cheap commercial models that Russian forces could easily jam and intercept. Mr. Rudominski, 26, had launched hardware start-ups before the war. Could he help?

The two friends set out to build an alternative system that could withstand Russian interference, while being simple to use and affordable enough to buy by the thousands. Himera, a company developing electronic warfare-resistant radios, was born.

Mr. Rudominski followed the start-up playbook, but in a war zone, with soldiers as beta testers. He took prototypes to the front, gathered feedback and refined his products.

The harder challenge was selling his radios to the army. Procurement officials trusted legacy defense firms, not a newcomer with a novel idea. “I got a lot of no’s,” Mr. Rudominski said.

But eventually, he got his products into the hands of special forces units, helping build credibility. Today, more than 10,000 Ukrainian soldiers use Himera’s technology, which has proved so effective that the U.S. Air Force has tested it as a potential acquisition.

Young people experimenting with drones ran into the same skepticism. Early in the war, Ukrainian officials focused on procuring the artillery, missiles, tanks and jets that had dominated warfare for decades.

“The government didn’t see drones as real arms,” said Kateryna Mykhalko, 25, who worked for a drone start-up in the war’s first year, then launched Technological Forces of Ukraine, a lobbying group for drone manufacturers. It began with five members in 2023 and has grown to more than 100 companies today, as Ukraine has become a global drone powerhouse.

The rise of youth in a sector this strategic was not universally welcomed.

Last year, Oleksiy Honcharenko, a lawmaker, questioned Ms. Mykhalko’s ability to lead an arms industry group at her age. “Can we look at her résumé?” he asked in a Facebook post. “Beautiful — yes. I see the photo shoots. But where’s the professionalism?” The criticism backfired, with politicians and civil-society figures rallying to her defense.

Ms. Mykhalko has since expanded her work beyond Ukraine, taking the helm of New Age Defense, which represents Europe’s leading drone manufacturers.

Many credit Mr. Fedorov, the freshly dismissed defense minister, with accelerating the generational shift.

He first served as Ukraine’s digital transformation minister, pushing beyond his official portfolio to champion defense entrepreneurs. His signature creation is Brave1, a government platform that funds defense start-ups and helps turn prototypes into battle-tested products. In three years, it has channeled hundreds of millions of dollars in grants and venture capital to companies, including Himera.

A recent Brave1 gathering in Kyiv felt more like a tech conference than a traditional defense exhibition. Young engineers in T-shirts crowded around small stands showcasing the latest drones, as techno music blasted through the venue. Upstairs, two hosts in their 30s taped a live podcast, bantering with start-up founders.

Artem Moroz, 29, who oversees Brave1’s investor relations, recalled his surprise when he joined Brave1 in 2024 and found colleagues speaking the language of Silicon Valley.

“It didn’t look like a government project,” he said. “It looked like a big tech company.”

When Mr. Fedorov became Ukraine’s defense minister earlier this year, he brought the generational shift into an old-guard bureaucracy largely led by Soviet-educated generals and bureaucrats.

Now, the ministry’s corridors are filled with young people like Mr. Antoniuk, whose employee found the mistake in Denmark’s aid commitments, and who steers Ukraine’s defense cooperation with foreign partners.

To do so, he has assembled a team of country managers, some younger than he is. What he looks for, he said, is fluency in foreign languages, sharp analytical skills and, above all, a capacity to adapt on the fly. “You have a lot of these qualities in young people,” he said.

Most of the people hired by Mr. Antoniuk, a Yale graduate, are also Western-educated and could have remained abroad, building safer, more lucrative careers. But with war raging at home, he said, only one path felt compelling: “Come back to Ukraine and help win this war.”

The question now is whether this generation will keep rising, or face the same resistance from an old guard that ended Mr. Fedorov’s tenure as defense minister. On Thursday, Mr. Rudominski and Ms. Mykhalko joined the protest in Kyiv against Mr. Fedorov’s dismissal, standing amid a crow of young people holding cardboard signs expressing their discontent.

Yeo Bondar, 21, held a sign reading, “This reshuffle is a step toward defeat.” She said Ukraine’s successful embrace of drone warfare had been driven by young people like Mr. Fedorov.

“We’re not excited to go back to the way the war was run previously,” she said. “People want to move forward, and Fedorov was the symbol of moving forward.”

Olha Konovalova contributed reporting.