SEOUL — Russia’s cyberoffensive against Ukraine has proved unprepared, uncoordinated and unable to overcome a well-prepared, flexible series of Ukrainian defenses that have relied on experience and expertise and confounded prewar predictions. Kyiv has been widely assisted by overseas information technology partners from the public and private sectors, and its ability to call on a dense network of civilian specialists has provided critical civilian-military coordination in cyberwarfare, experts said.

Cyberwarfare was a leading topic of conversation at a major conf …