<p>On 21 April 1964, SAC passed a milestone, due mostly to retirement of the very numerous Boeing B-47 Stratojet: the number of ICBMs on alert surpassed the number of bombers on ground alert. From that day, the ICBM alert force was to outdistance the bomber alert force. Titan II and Minuteman ICBMs were growing in number. The numbers kept going up even when all first-generation, liquid-fueled Atlas E/F and Titan I missiles were retired in a four-month surge of activity in early 1965.
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<p>The defense of the North American continent traditionally enjoyed a priority equal to SAC's, and ahead of everything else. The Air Defense Command was much reduced in size from its heyday (in 1954) when it boasted 55 squadrons of all-weather interceptors. Still, ADC had 21 squadrons in the field in 1964 when the final aircraft manufactured as an interceptor, a Convair F-106A Delta Dart, emerged from a factory update programme. Newer fighters were soon to be reassigned from other duties to the interceptor role, but it was increasingly clear that Moscow lacked a bomber force like SAC's. What the Soviets did have was a force of intercontinental ballistic missiles which grew from a few dozen in 1960 (the time of the much talked-about but nonexistent 'missile gap') to as many as 2,000 a decade later.
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<p>This was the era in which measures to bring the nuclear genie under control forced a modicum of cooperation between Washington and Moscow (the 1963 ban on atmospheric testing of nuclear weapons, for example) but the Cold War remained icy, and the missile threat frightened most Air Force planners more than anything else. It was conventional wisdom in the mid-1960s—and remained so, unchanged until the mid-1980s—that there was no defense against ICBM attack.
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<p>Belatedly, McNamara replaced the policy of assured destruction with a policy of flexible response. The concept of flexibility was meant in a most serious way. If the Soviets attacked in Lebanon, the United States might find it convenient to retaliate with an attack on Vladivostok. The policy allowed for a wide range of 'limited war' situations, including (previously unthinkable) limited nuclear war. If the Soviets obliterated Chicago, the United States would flatten Magnitogorsk, but the response would be proportionate to the provocation and might not lead to an all-out exchange. With modifications, flexible response has remained US policy ever since.
