WASHINGTON—Ukraine’s weekend attacks against military airfields deep inside Russia signal the long-stalemated war is entering a perilous phase, with both sides seemingly intent on escalation and prospects for a U.

Costing as little as $400 apiece, Kyiv’s flying machines are successfully neutralizing sophisticated Russian equipment worth thousands of times more

A surprise Ukrainian drone attack that targeted Russian air bases hosting nuclear-capable strategic bombers was unprecedented in its scope and sophistication.