This chapter presents a realistic thought experiment on conflict in cyberspace and analyses it from two different perspectives: information security and national defence. The analysis as well as empirical findings show a major defence anomaly: Western armed forces do not defend homeland targets against foreign cyber attacks. This overlooked defence anomaly is the cause of the profound cyber insecurity in the West. To understand and mitigate the problem, the author relies on insights from theoretical knowledge on defence adaptation, produced over decades of security studies research. Successful defence adaptations have depended on concepts and organisations rather than on technology alone. To get cybersecurity right, the West needs a new theory of victory, one that suits the realities of cybered conflict. Such a change will not simply arise from the armed forces. While the “Iron Dome” system had caught the attention of studies focused on technology and innovation, the author has analysed its development and adoption as a defence adaptation process. The “Iron Dome” demonstrates that both external pressure on defence organisations and insider champions of change are essential for a peacetime strategic defence adaptation to succeed.