Late one evening this month, two Ukrainian commandos eased onto a side street in Kyiv in a battered SUV. Back from a dangerous nighttime assault on Russian positions in the Crimean peninsula, they slipped into a sparsely furnished apartment where they sat at desks, weary and a little disheveled, and described their latest operation in matter-of-fact fashion.

"Very tough,” said Askold, 38. "It was our most difficult operation yet,” added Kukhar, 23. Members of a unit in the special operations forces of GUR, Ukraine’s military intelligence service, the men gave only their call signs in accordance with military protocol.

The two men had joined more than 30 others racing more than 160 kilometers (100 miles) across the western Black Sea on jet skis to attack critical Russian defense installations before making their getaway, the second Ukrainian amphibious raid in six weeks.