Cognitive warfare manipulates environmental stimuli to control people's mental states and behaviors. Through widespread smart devices and social media platforms, digital authoritarianism has disseminated disinformation to intensify existing social conflicts in democracies. Not only did Russia meddle with the UK's Brexit vote and elections in the Baltic states, but China has also interfered in Taiwan, Australia, and New Zealand and has highlighted racial conflict in the US. As freedom of expression is often abused, cognitive warfare seems to be an emerging threat to democracy that needs to be seriously dealt with. However, on the other hand, human prejudice is persistent and somewhat immune to belief revision. Given this fact, has the harm of cognitive warfare been exaggerated? Also, while the illusory truth effect has been repeatedly found in psychological labs, political scientists show that voter behaviors are not always steered in reality in light of fake news. So why is there a gap between input and output? Does the filter bubble effect exist? In this talk, I will explain how cognitive warfare works and how to reduce its harm.