debate over the merit of ABMs began. Difficulties that had already made an ABM system questionable for defending against an all-out attack. One problem was the Fractional Orbital Bombardment System (FOBS) that would give little warning to the defense. Another problem was high altitude EMP (whether from offensive or defensive nuclear warheads) which could degrade defensive radar systems. When this proved infeasible for economic reasons, a much smaller deployment using the same systems was proposed, namely Safeguard (described later). Defense against MIRVs ABM systems were developed initially to counter single warheads launched from large intercontinental ballistic missiles (ICBMs). The economics seemed simple enough; since rocket costs increase rapidly with size, the price of the ICBM launching a large warhead should always be greater than the much smaller interceptor missile needed to destroy it. In an arms race the defense would always win. In practice, the price of the interceptor missile was considerable, due to its sophistication. The system had to be guided all the way to an interception, which demanded guidance and control systems that worked within and outside the atmosphere. Due to their relatively short ranges, an ABM missile would be needed to counter an ICBM wherever it might be aimed. That implies that dozens of interceptors are needed for every ICBM since warhead's targets couldn't be known in advance. This led to intense debates about the "cost-exchange ratio" between interceptors and warheads. Conditions changed dramatically in 1970 with the introduction of multiple independently targetable reentry vehicle (MIRV) warheads. Suddenly, each launcher was throwing not one warhead, but several. These would spread out in space, ensuring that a single interceptor would be needed for each warhead. This simply added to the need to have several interceptors for each warhead in order to provide geographical coverage. Now it was clear that an ABM system would always be many times more expensive than the ICBMs they defended against. Anti-Ballistic Missile Treaty of 1972 Technical, economic and political problems described resulted in the ABM treaty of 1972, which restricted the deployment of strategic (not tactical) anti-ballistic missiles. By the ABM treaty and a 1974 revision, each country was allowed to deploy a mere 100 ABMs to protect a single, small area. The Soviets retained their Moscow defences. The U.S. designated their ICBM sites near Grand Forks Air Force Base, North Dakota, where Safeguard was already under advanced development. The radar systems and