Abstract: Effective defenses that are designed to protect civilians in war have significant implications for policy planning, military strategy, international relations, domestic politics and economics. Defenses can increase or decrease overall humanitarian welfare. Surprisingly, existing legal scholarship has focused almost exclusively on offensive action, failing to consider the effects of defenses on the strategic interactions between armed rivals or the humanitarian consequences of defenses. The implications of defenses for the interpretation and application of the international legal rules on the use of force have also gone largely unexplored. We set out to fill this significant gap. We study the operation of defensive systems in both asymmetric rivalries and symmetric rivalries, and consider the interplay between defenses and offensive measures. We analyze how defensive systems are likely to affect parties’ wartime conduct and the potential consequences for the welfare of civilians on both sides of the conflict. A central motivating observation is that defenses have the potential of safeguarding not only the lives of the defending party’s civilians but also those on the opposing side. Our analysis further considers how international law, and especially the principle of proportionality, might affect parties’ choices with regard to investments in defenses. Counter-intuitively, we caution that under some circumstances, an overly-restrictive application of the principle of proportionality might deter investment in defenses, thereby decreasing overall humanitarian welfare. To make our theoretical models more concrete, we draw on several real-world examples: the Israeli anti-ballistic missile system, Iron Dome; the deployment of anti-missile defenses by Japan and the United States to meet the threat from North Korea; and the race between the two Cold War protagonists to develop superior inter-continental anti-ballistic missiles systems, which eventually lead to the 1972 ABM Treaty.