Mr. Khanko is a veteran of the war that Russia whipped up in Ukraine’s east in 2014 to hinder its neighbor from integrating with the West. While the U.S. and its allies have been fretting that Mr. Putin will order a forceful military thrust to rein in Ukraine again, Mr. Khanko has been laying plans to send his wife and small child westward so he can wage a partisan war from the woods around Poltava.

“Even if they get to Poltava, they won’t be here for long,” said Mr. Khanko, who sports a buzzcut and long black beard…

[T]here are thousands of veterans in this region alone, and while the powerful Russian army would likely overrun Ukrainian forces, holding the territory would come at a huge cost, Mr. Khanko said. A recent national survey by a Kyiv pollster showed that one-third of Ukrainians are willing to take up arms if Russia launches an all-out war.

“I know what I am fighting for, but how will Putin sell it to Russians when tens of thousands of graves appear across the country?” Mr. Khanko said. “For what?”