The roiling water can be treacherous, the banks are steep and slick with mud, and the riverbed is covered in jagged, hidden boulders.

Yet Ukrainian border guards often find their quarry — men seeking to escape the military draft — swimming in these hazardous conditions, trying to cross the Tysa River where it forms the border with Romania.

Lt. Vladyslav Tonkoshtan recently detained a man on the bank, where he was preparing to cross the river in the hope of reuniting with his wife and children, whom he had not seen in two years since they fled to another country in Europe.

That thousands of Ukrainian men have chosen to risk the swim rather than face the dangers as soldiers on the eastern front highlights the challenge for President Volodymyr Zelensky as he seeks to mobilize new troops after more than two years of bruising, bloody trench warfare with Russia.

“We cannot judge these people,” Lieutenant Tonkoshtan said. “But if all men leave, who will defend Ukraine?”

With Russia having seized the initiative on the battlefield in recent months, Ukraine’s ability to defend itself hinges on replenishing its arsenal of weaponry, a matter largely up to allies, and mobilizing troops at home.