In a forest in western Ukraine, a few dozen young men and women stood at attention in two lines in the fading evening light. Some had fake guns slung over their shoulders.

Among them was Olesya Vdovych, who had spent the day with other members of the scouting organization Plast, hauling logs, running drills and learning about first aid as part of a two-week camp last August.

“I’m eager to be prepared,” Ms. Vdovych said at the time, her long blond hair tied in two braids under a forest green cap. With a number of her friends and family fighting in the war against Russia, she said she felt it was important to be ready for any situation.

For young Ukrainians like Ms. Vdovych, the once-carefree summers of childhood and young adulthood were forever altered by Russia’s invasion of the country more than two years ago. Since then, the war has ground on with little change in Ukraine’s fortunes, even as Ukrainian forces made a rare incursion into Russia last week, taking some territory, and engaging in furious fighting with Russian forces. Ukraine has struggled to hold back Russian forces in the east, and devastating airstrikes continue to bombard cities far from the front lines. In April, Ukraine lowered the draft age for young men, to 25 from 27.