The intensity of the reaction resulted from the strategic implications of the president's plan and from the daunting complexity of an undertaking to erect a missile-proof shield over the United States and its allies. The Strategic Defense Initiative (SDI) calls for the use of new technologies to destroy attacking missiles and warheads before they reach their targets. Sensors in space would track enemy missiles from the instant they were launched. Politically, SDI will poison US-Soviet relations, critics say, and increase the risk of nuclear war. The present condition of mutual, stable deterrence will be undermined and replaced with uncertainty and ambiguity. The decline in popular support for offensive nuclear deterrence in the Western democracies and in particular for the replacement of existing weapons with more modern ones also contributed to the Reagan administration's decision to initiate SDI.