Russia bombarded Ukraine’s second-largest city with massive strikes in the early hours of Saturday, its mayor said, one night after Moscow carried out one of the war’s largest aerial assaults on Ukraine.

Former Rep. Chris Stewart, a former fighter pilot, says the U.S. isn’t prepared for this change in modern warfare.

The attack on Friday, which killed at least four people nationwide, came two days after Russia vowed retaliation for an audacious drone assault on its strategic bomber bases.

PRYLUKY, Ukraine (AP) — At least five people, including a 1-year-old child, his mother and grandmother, were killed Thursday in a nighttime Russian drone attack on the northern Ukrainian city of Pryluky, officials said.

When the chief of the local fire department was called to a scene of a Russian strike in the central Ukrainian city of Pryluky overnight, he and his brigade found five people were killed and nine injured after a drone hit a residential building.

The attack injected some much-needed optimism into Ukraine, beleaguered by years of war and worn down by what many see as U.S. pressure to concede to Russian demands.