the Soviet Union (A-35 anti-ballistic missile system) and the United States (LIM-49 Nike Zeus) developed anti-ballistic missile systems. Had such systems been able to effectively defend against a retaliatory second strike, MAD would have been undermined. However, multiple scientific studies showed technological and logistical problems in these systems, including the inability to distinguish between real and decoy weapons. MIRVs MIRVs as counter against ABM The multiple independently targetable re-entry vehicle (MIRV) was another weapons system designed specifically to aid with the MAD nuclear deterrence doctrine. With a MIRV payload, one ICBM could hold many separate warheads. MIRVs were first created by the United States in order to counterbalance the Soviet A-35 anti-ballistic missile systems around Moscow. Since each defensive missile could be counted on to destroy only one offensive missile, making each offensive missile have, for example, three warheads (as with early MIRV systems) meant that three times as many defensive missiles were needed for each offensive missile. This made defending against missile attacks more costly and difficult. One of the largest US MIRVed missiles, the LGM-118A Peacekeeper, could hold up to 10 warheads, each with a yield of around —all together, an explosive payload equivalent to 230 Hiroshima-type bombs. The multiple warheads made defense untenable with the available technology, leaving the threat of retaliatory attack as the only viable defensive option. MIRVed land-based ICBMs tend to put a premium on striking first. The START II agreement was proposed to ban this type of weapon, but never entered into force. In the event of a Soviet conventional attack on Western Europe, NATO planned to use tactical nuclear weapons. The Soviet Union countered this threat by issuing a statement that any use of nuclear weapons (tactical or otherwise) against Soviet forces would be grounds for a full-scale Soviet retaliatory strike (massive retaliation). Thus it was generally assumed that any combat in Europe would end with apocalyptic conclusions. Land-based MIRVed ICBMs threaten MAD MIRVed land-based ICBMs are generally considered suitable for a first strike (inherently counterforce) or a counterforce second strike, due to: Their high accuracy (low circular error probable), compared to submarine-launched ballistic missiles which used to be less accurate, and more prone to defects; Their fast response time, compared to bombers which are considered too slow; Their ability to carry multiple MIRV warheads at once, useful for destroying a whole missile field or several cities with one missile. Unlike a