Anti-ballistic missile (ABM) systems of the time only attacked the enemy nuclear warheads after they were released by ICBMs. As a single ICBM could carry as many as a dozen warheads, dozens of defense missiles were required per attacking missile. A single Excalibur contained up to fifty lasers and could potentially shoot down a corresponding number of missiles, with all of the warheads still onboard. A single Excalibur would require dozens of ICBMs to take down, dramatically reversing the cost-exchange ratio that had previously doomed ABM systems.