The last time Michael Hudson saw Nick Nikonov alive, the wiry Ukrainian had just bested him in a fight, bruising his ribs and jamming his thumb.

It was 2015, and Hudson, an officer in the U.S. Marine Corps, was preparing for a deployment to Afghanistan. Nikonov, an officer in the Ukrainian military, was in the United States for training. He was intensely competitive and loved to spar, Hudson recalled, reflecting on their friendly bout.

Though hardened by the fighting he’d seen in Ukraine’s war with Russian-backed separatists, Nikonov had a softer side, too. He was a family man, Hudson said, recounting how his friend brought toy dolls from Ukraine as gifts for Hudson’s daughters and took it upon himself to look in on them while Hudson was overseas. “Just like any Marine you would trust,” he said in an interview.

Nikonov was killed last week battling Russian forces around Mariupol, a city in southeast Ukraine that’s seen withering violence. In a Facebook post paying tribute to his friend, Hudson called him a “true warrior and hero.”

U.S. forces train with foreign militaries, trading skills and experience, so their personnel can better work together in operational settings or times of crisis. But this has another effect just as elemental: It sparks relationships in unlikely places, such as the shores of the Black Sea, where a U.S. Marine who grew up in Northern California and a lanky officer from Odessa would become lasting friends.

Nikonov, a lieutenant colonel in an elite Ukrainian naval unit similar to the U.S. Navy SEALs, was offered numerous opportunities to serve away from the front lines “but passionately refused,” Hudson wrote online.

“As a leader, Nick led from the front and cared deeply for his teammates,” he added.

With Mariupol under heavy bombardment, Hudson organized a fundraiser intended to purchase medical supplies for his friend’s unit. Nikonov said that while he appreciated the support, it would be difficult for anything to make it inside the city. He and his troops were surrounded, Hudson said.

“It’s a huge, huge loss,” Hudson said. “He’s the kind of guy I want to be led by.”

The war, which began three weeks ago, has produced thousands of military and civilian casualties, but it’s been difficult to obtain a thorough, accurate picture of the true human toll on either side as both work diligently to shield the disclosure of information that could prove advantageous to the other.

Ukrainian President Volodymyr Zelensky said Saturday that 1,300 service members had been killed, but The Washington Post has been unable to verify a number. Independent analysts have said the number is potentially higher, though, noting both countries may obscure the true cost.

“Military casualties are inherently difficult to track, especially in a conflict like this where the information front is so important,” said Margarita Konaev, the associate director of analysis at Georgetown University’s Center for Security and Emerging Technology. “There is an incentive to overestimate and overcount some things,” like enemy troops killed, and underestimate and undercount other things, like their own casualties, she said.

A spokesperson for Ukraine’s Defense Ministry confirmed Nikonov’s death but referred additional questions about the incident to the armed forces, which did not return a request for comment.

Russian casualty counts have been similarly murky, with U.S. intelligence estimates putting the number last week at between 2,000 and 4,000 dead but noting there was “low confidence” in those numbers.

Pentagon estimates of total Russian soldiers taken out of the fight point to many more wounded, captured or missing, saying late last week that Ukraine and Russia each have lost about 10% of their combat power. The Russians have poured more than 150,000 troops across the border. Ukraine’s active-duty military is estimated to number about 200,000.

As causalities mount, so too do worries among American military personnel and veterans who have formed strong friendships with their colleagues in places such as Ukraine, where U.S. and other Western militaries have trained alongside local forces for years.

Hudson, who served in elite Marine Corps reconnaissance units before retiring from the service in 2018, met Nikonov more than a decade ago and said that he stood out as unusually talented, with equal parts competence and a profound affection for the troops he led.

“I noticed those attributes with Nick in the first 30 seconds,” Hudson said. “There is a deep bond formed right off the bat.”

While Hudson and his fellow Marines were deployed to Ukraine, Nikonov showed them how to explore the culture, he said, including in his hometown of Odessa, the beating artistic heart of the country. Nikonov helped them score tickets to a showing of “Swan Lake” at the world-famous opera and ballet house now fortified with sandbags.

Their military ties grew more complex after Russia annexed Crimea in 2014 and separatists backed by the Kremlin seized territory in the Donbas region. Nikonov and his unit fought along the eastern front, Hudson said, recalling a story from his friend about an attack the Ukrainians had carried out on an enemy convoy that resulted in a sizable recovery of ammunition and other supplies.

Nikonov dug through the flaming wreckage and pulled out a separatist uniform for Hudson as a battlefield souvenir. It had burn marks, Hudson said, and he wore it during his down time on a subsequent tour in Afghanistan, cherishing the gift from one newly minted combat veteran to another.

His death has shaken close friends, Americans and Ukrainians alike, who have taken to Nikonov’s Facebook page to offer condolences. Many include images of him in one of his favorite places: in the air over Ukraine’s southern coast. In one undated video, Nikonov explains he is parachuting onto the Kinburn Peninsula, where the Dnieper River and Black Sea converge. “I love you, and looking forward to seeing you,” he says.

Hudson has not yet made contact with Nikonov’s immediate family, including his wife and two children, but said he intends to help them if needed. Donations to the Marine Reconnaissance Foundation are still flowing to help Nikonov’s unit get medical supplies. The nonprofit assists U.S. military veterans and their families, but Nikonov, having completed the Marine Corps’ grueling reconnaissance training course alongside his American counterparts, made such distinctions irrelevant, Hudson said.

“We consider him one of our own,” he said.

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The Washingon Post’s Serhiy Morgunov in Lviv, Ukraine, and Dan Lamothe in Washington contributed to this report.