KOROSTEN, Ukraine—A 20-year-old Ukrainian soldier on the front lines spent weeks reassuring his mother about the firefights waged last year in eastern Ukraine.

On Dec. 23, he revealed the true danger during a last call home. “It’s hot,” Ihor Tychyna told his mother. “Very hot.”

Four days later, a sniper’s bullet pierced his head and put him...

KOROSTEN, Ukraine—A 20-year-old Ukrainian soldier on the front lines spent weeks reassuring his mother about the firefights waged last year in eastern Ukraine.

On Dec. 23, he revealed the true danger during a last call home. “It’s hot,” Ihor Tychyna told his mother. “Very hot.”

Four days later, a sniper’s bullet pierced his head and put him into a coma. He died at a hospital far from home on New Year’s Day.

Ukrainians aren’t waiting for the war with Russia to start. They have been fighting one since 2014, when Russia first invaded and seized portions of the country. Moscow has since used armed proxies in eastern Ukraine to pursue a grinding conflict aimed at bringing its former vassal to heel. The war has cost the lives of at least 14,000 people, drained billions of dollars from Ukraine’s economy and displaced hundreds of thousands of residents.

Even a partial withdrawal of Russian forces now threatening a larger invasion likely wouldn’t mark the end of a long-running battle over the control of Ukraine. Russian President Vladimir Putin faces a generation of Ukrainians who have already turned against their powerful neighbor. Young men, barely teenagers in 2014, have come of age and taken up arms.

One of them was Mr. Tychyna, a volleyball standout who had hoped to be a computer programmer after serving in the army. He was among Ukraine’s 4,500 military service members killed in the nation’s war, nearly double the U.S. military deaths over two decades in Afghanistan.

“We’ve been stewing in this war for so long,” said Taisiya Budzynska, one of Mr. Tychyna’s schoolteachers. “This war has made Ukraine a nation.”

Most of the Ukrainian military casualties came in the first year, when fighting was heaviest and training and medical provision at their weakest. One or two have died every week over the past few years, often by sniper or mortar fire across the front lines, a divide largely fixed in a 2015 deal that ended the heaviest fighting but didn’t bring peace.

The war feels far off to those living in Korosten, Mr. Tychyna’s hometown. The city of some 60,000 is hundreds of miles from the front lines. Yet his death brought its main street to a halt on Jan. 6. Roughly 600 people joined the funeral procession and many knelt as his coffin was carried to his memorial service.

A video of Ihor Tychyna.

Mr. Tychyna wasn’t a zealous nationalist, as Moscow portrays its rivals in Ukraine. He wore his patriotism lightly and came to see the fight against the Russians as his duty and the military an honorable profession, acquaintances said.

The spark of patriotism first flashed in his boyhood during demonstrations against pro-Russian President Viktor Yanukovych in the winter of 2013. Ms. Budzynska recalled how young Ihor’s class would play a popular anthem of the protests, “Warriors of Light,” and wave around the flashlights on their cellphones.

Mr. Yanukovych eventually fled, and Russia seized the Crimean peninsula, later sending agents, arms and fighters to whip up a separatist conflict in Ukraine’s east.

Mr. Tychyna’s father, Oleh Tychyna, a former marine, wanted to join the fight against Russia, but recruiters wouldn’t agree to send him to the front. “What war? You have three children,” Iryna Tychyna said they told her husband.

Early on, Ukrainian forces retook the strategic city of Slovyansk, a victory that had a special resonance in Korosten. After the 1986 nuclear accident in nearby Chernobyl, children went to a lakeside resort in Slovyansk for rehabilitation.

The tide turned late that summer. Russia covertly sent in military units that overwhelmed Ukrainian forces in parts of the east and carved two separatist regions from government control.

Natalia Chyzhevska, the deputy mayor of Korosten.

War stories

Post-Soviet life hasn’t enriched residents of Korosten, a low-key transport hub known for its potato-pancake competition. The city lost tank and helicopter regiments based there. Industrial plants linked to Russia’s defense industry closed. A porcelain factory shut, losing out to inexpensive Chinese products.

Yet people managed to get by, some trading farm products across the nearby border with Belarus, and others working for a local fuel company or at wood-processing plants, which supplied such customers as furniture retailer IKEA.

In January 2015, Korosten lost its first native son in combat at the airport in Donetsk, which became a stronghold of Russian-led separatists. The death shook the city, which renamed a street after him.

One of Ihor Tychyna’s former classmates looking at a memorial wall dedicated to Ukraine's independence.

“It was hard to grasp,” said Korosten’s deputy mayor Natalia Chyzhevska, who had been a schoolmate of the soldier. “You are walking along a street named after a person who sat on the bench next to you at school.”

Young Ihor’s father died suddenly that year, leaving his mother, a nurse, to care for the boy and two sisters. The boy’s teacher said he turned from a joyful student to one more introverted. He grew past 6-foot and doted on his younger sister. He and his family closely followed the news from the front lines. “He absorbed it like a sponge,” his mother said. She saw his eyes light up at stories of marches and jumps told by one relative, a former paratrooper.

Ms. Budzynska said she would remind her pupils of a Ukrainian nationalist leader who said anyone who chooses bread over freedom would lose both. She took Mr. Tychyna’s class to see a movie about the defenders of the airport in Donetsk. The soldiers were known as the “Cyborgs” for fighting off waves of attacks, before their retreat.

On the way back from the movie theater, Ms. Budzynska recalled the boys asking, “Why is the world not reacting to what is happening?” she said. She told them Ukrainians would have to rely on themselves.

Taisiya Budzynska, Ihor Tychyna's former teacher in Korosten.

‘Don’t go’

Mr. Tychyna enlisted in the army immediately after high school graduation. Beyond his feelings of duty, Mr. Tychyna saw the military as a path to a better financial future, said Oleksiy Bondar, a close friend. Mr. Tychyna’s mother pleaded with him to wait a couple of more years and help her with his sisters.

He joined the 95th Air Assault Brigade, based in the regional capital Zhytomyr. He was first deployed to a checkpoint that marked the limit of Kyiv’s control where the Crimean peninsula extends from mainland Ukraine.

Ms. Tychyna said her son was under constant pressure from the Russian side and saw a psychologist. Last fall, he was transferred some 500 miles to the east. “Ihor, don’t go. It’s very dangerous,” his mother recalled saying. He told her not to worry, that he would be OK.

Mr. Tychyna operated an automatic grenade launcher on the front lines near the city of Horlivka. He assured his mother in short calls that everything was all right. On Dec. 23, he let slip the intensity of the fighting, and his mother cried. “Ihor, take care of yourself,” she told him.

He was struck by the sniper’s bullet around 3 a.m. on Dec. 27.

Mr. Tychyna was evacuated to the city of Kharkiv. The news quickly reached Korosten. Those at his old school learned it from a girl whose brother worked at the recruiting office. City Hall officials heard it from a veterans organization.

The housing where Ihor Tychyna grew up and where his family still lives in Korosten, Ukraine.

The whole town fretted, and Mr. Tychyna’s fight for survival was the talk of year-end company parties. News of his death on New Year’s Day pierced the celebrations. His was the first Ukrainian combat fatality of 2022.

The funeral service took place six days later on the city’s central square.

Korosten now worries about a bigger war. Belarus, which is carrying out exercises with its close Russian ally, is a few dozen miles north.

Ms. Budzynska, the schoolteacher, said she was shocked recently to see people buying staples ahead of a possible invasion. Her father, born in 1948, during difficult postwar years, urged her to do the same.

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On a recent day, his mother trudged through snow to her son’s grave.

Ms. Tychyna gestured to the spot where her son is buried next to her husband. “Oh, God,” she said. “That was supposed to be my place.”

She brushed snow off the cross, then collapsed, wailing, onto wreaths held together by tape against a biting wind.

“My Ihor,” she said, sobbing. “My son. My son.”

A memorial honoring Ihor Tychyna at his former school in Korosten, Ukraine.

Write to James Marson at james.marson@wsj.com