Active defense protects against theater missiles by destroying them in flight. Engagement capability is required throughout all phases of the missile’s trajectory (boost, post-boost, mid-course, and terminal) to prevent saturation of point defense, to negate warhead effects, and to ensure minimal leakage in defending critical assets. Therefore, active defenses must consist of defense in depth to provide multiple engagement opportunities with differing technologies, increasing the probability of kill, and countering the enemy’s counter-measure efforts. Active defenses could consist of space-, air-, ground-, and sea-based systems. If a strategic ballistic missile defense system is deployed, the active TMD should be supported by, but not limited by, those systems to increase the defense in the theater of operations. Active defense is considered one of the four pillars of TMD capability. (JCS J-38 CONOPS)