He left Russia to fight Putin. Now he leads Ukrainian soldiers in combat against his former compatriots.

DONETSK REGION, Ukraine — The ground below the Ukrainian positions was scorched black, burned by flares dropped from Russian jets. The green wheat fields beyond were pockmarked with craters gouged out of the earth by Russian artillery strikes.

“This was such a beautiful scene,” the unit commander said, looking out across the rolling countryside Friday morning, “and they ruined it, the swine.”

The commander, who asked to be identified only by his code name, Kandalaksha, leads a volunteer unit camped out in the hills of eastern Ukraine. For two months the unit has been holding part of the line south of the city of Izium, blocking a Russian offensive to encircle and seize the eastern Donbas region.

Kandalaksha is something of an anomaly. He is from Russia, and describes himself as a political refugee. An opponent of President Vladimir V. Putin’s government, he left his homeland in 2014 when Moscow annexed Crimea and began supporting a separatist war in the eastern regions of Luhansk and Donetsk.

“I was fighting the Putin regime,” he said, “and I understood the hottest place to fight against the Putin regime was in Ukraine.”

Soon after arriving in Ukraine he took a step beyond political activism and joined a volunteer military unit in 2015. “I was searching myself and I looked for a way to be useful,” he said. “I thought it would be most honest to go to fight for the country.”

When Russia launched a full-scale invasion of Ukraine in February, he volunteered again and fought in the northwestern suburbs of the capital, Kyiv, for a month. Then as Russia withdrew from Kyiv and concentrated its forces on the battle for the Donbas, his unit was also sent east.

“We don’t see them but they shell us,” he said of his erstwhile compatriots, the Russians who are positioned about 10 miles away from his outpost. “Rarely a day goes by without shelling. They try to bite us, but our forces are holding their positions and are not letting them advance.”

Ukrainian forces are under increasing pressure in eastern Ukraine as the Russian military has switched tactics. It has focused its forces and firepower on a much smaller target with a more limited goal: encircling a last crescent of towns and villages that belong to Donetsk and Luhansk.

Every few days soldiers from this unit of the 95th Air Assault Brigade head to the frontline, which they call ground zero, giving others a break from the pounding artillery. The soldiers are caustic about the type of warfare they are undergoing on the open country of eastern Ukraine. They describe themselves as cannon fodder, and reduced to “cotton” or stuffing under the heavy barrages of artillery.

But their morale seems high and, as volunteers, most said there were convinced of the need to stand up to Russian aggression.

One of the volunteer soldiers is a theater director, another a university economics lecturer.

“It’s much more difficult sitting and doing nothing,” said the lecturer, who goes by the code name Academic.

Maksim Bulgakov, 40, the theater director, said he had never wanted to join the army. “My father, brother and grandfather were artillery officers, but I never wanted to be,” he said. “But it’s such a time. You have a problem and you have to decide.”

The men, and one woman, lie low, sleeping in a farm building and keeping out of sight of Russian drones during the day. They operate artillery guns from the tree lines in the area but did not allow visiting reporters to see them in use.

Russian planes have bombed the area, leaving enormous craters 10-feet deep and damaging some hamlets and farms. An artillery shell landed close but the few soldiers at the outpost seemed unconcerned, cleaning weapons and chopping wood under the trees.

They sleep on wooden boards and camping mats, sharing the barn with two small cats; they named one of them Hitler because of a smudge of black on its face that recalls the Nazi leader’s toothbrush mustache. The cats clamber over the sleeping bodies, as soldiers come and go throughout the night, taking turns to do a few hours on guard duty.

The commander, Kandalaksha, takes a turn on duty too. “Our spirits are high,” he said. “All the men fighting understand that the whole world depends on Ukraine right now. We will do what we can.”

A trained electrical engineer from Murmansk in Russia’s far north, the commander became interested in politics around 2008 or 2009 when he saw a video by the opposition activist and politician Alexei Navalny. The segment exposed the corruption and embezzlement of billions of dollars of state money by the Russian leadership.

“After that I understood all of this money is going to the president and top-level people,” he said. “I began to ask questions and became quite active. He began distributing leaflets and evaded police road blocks to take part in a large protest rally in Moscow at the time of legislative elections in 2011.

But he soon came under scrutiny from Russia’s secret service. He worked at a hydroelectric plant but felt his political activity caused the leadership to deny him a promotion. “They wanted me to go,” he said.

He found a job in southern Russia in 2013 and when the democracy protests began in Ukraine — ultimately leading to the ouster of President Viktor Yanukovych — he began to contemplate leaving Russia altogether. Family members were opposed to his move, but they understood, he said.

He asked that his family members not be identified for their own protection.

He said he did not regret leaving. “I don’t think I will ever go back,” he said. “I feel very good here. I am at home.”

He is both cynical and hopeful about the possibility of change in Russia. He said Mr. Putin had calculated that the West would not oppose his imperial ambition.

“His hunch was that he would face little response,” he said. “But if you fight him, everything is possible.”

“This is the moment when a lot of things are going to be resolved,” he added.

He does not believe that a change of leadership would change anything. “If Putin goes, the system will stay,’’ he said. “We need to change the system.”

He said he was appalled at recent comments by former Secretary of State Henry Kissinger suggesting that Ukraine cede territory in a potential peace agreement with Russia. An editorial in The New York Times was interpreted as suggesting the same.

“That is a horrendous thought,” he said. “The whole world has to destroy the Russian cancer. It is the quintessence of evil and should be defeated by all humanity.”

He said large-scale Western support for Ukraine would help change minds in Russia as people would see the improvements and development of freedoms. The youth in Russia already understood how unjust their system was, he said.

“I hope Russia will change and cease being like it is now,” he said. “It’s not that I want it to be destroyed but I hope Russians will change their minds.”