KYIV, Ukraine — From a hotel on a posh street in Barcelona, a 22-year-old Ukrainian in a red beanie and big black headphones is helping take down some of Russia’s most powerful websites — including state media and even the official page of the Kremlin.

Alex Horlan would prefer to be at home, preparing to confront the Russian invasion of his homeland with a gun. But when he got stuck in Spain after Ukraine’s airports closed last week, he tapped into his expertise in cybersecurity to launch his own war against Russia online.