Russia's invasion of Ukraine, initiated on February 24, 2022, is developing in a very different way from Russia's annexation of Crimea in 2014. While Ukraine was unable to deter Russia's hybrid war at the time, it now successfully steps up its military response by rallying domestic and foreign public opinion. The causes for this can be explained when focusing on NATO's concept of "cognitive warfare." Based on the concept of "cognitive warfare," this article examines how cognitive warfare works in the Russia-Ukraine war and how it influences both sides, and then derives its implications for the ROK defense.