KYIV, Ukraine—It was a scorching summer day in 2014 when Artem Kravchenko and hundreds of other Ukrainian soldiers put their trust in a Russian promise not to open fire and retreated in a column from the surrounded city of Ilovaisk. By the end of the day, Kravchenko found himself lying in a ditch surrounded by dead comrades after Russian forces shot up the column. With three Russian bullets in his body, he had to drink his own urine to survive. “They started shooting from the right, so we went to the left and met another ambush there,” said Kravchenko, who...