During the 1950s the USA military started to worry that Russian bombers might fly over the Arctic and Canada and drop nuclear bombs on American cities.
The decision to drop atomic bombs on Japanese cities did little to shorten the war but it did create an existential threat to the USA. Several things followed from the American’s demonstration that nuclear bombs worked, that they had some and were prepared to drop them on civilian targets.
1 Other countries would have to have such bombs or be prepared to obey America. Sure enough, in 1949 The Soviet Union detonated Joe -1, its first atomic bomb. Five years before the CIA expected the Soviet Union to have a A bomb. The bomb was about Hiroshima size.
2 Bombs would get more powerful. In 1952 the USA tested a 10.4 megaton hydrogen bomb, but it was too heavy to be deliverable. In 1953 the Soviet Union tested a hydrogen bomb which had a much lower yield but was deliverable. Soon both sides had multimegaton hydrogen bombs which were deliverable.
Until the US introduced the world to the nuclear age America had been effectively immune to aerial bombing. Now it wasn’t and the bombs were a lot more powerful. A successful attack could turn the USA into a third world country.
The US military realised that America was almost defenceless. In a simulated Soviet attack [Operation Tailwind], 94 SAC bombers tested the air defence system of the continental USA by flying over Canada at night, and using electronic countermeasures to simulate a Soviet raid. Only 7 of the planes were spotted by radar and “shot down.
America had no reliable way of detecting approaching Soviet bombers. If incoming bombers were detected there was no good way of destroying them.
It might not even know it had been attacked. If Los Angeles was destroyed or the SAC base at Homestead in Florida, how long would it take for Washington and SAC at Omaha to realised what had happened? Probably too long. Even more worrying; what if the USA wrongly believed it was being attacked and started WW3?
In 1961, when tensions were very high during the Berlin Crisis, SAC headquarters in Omaha lost contact with the Thule radar base in Greenland. A SAC officer called NORAD headquarters in Colorado Springs to find out what was wrong. The line was dead. Both Thule and NORAD were considered key bases that were likely to be hit very early in any Soviet attack. That they were both offline strongly suggested that an attack was in progress. A technical fault seemed unlikely since one base was east of Omaha and the other to the west.
It later emerged that a single AT & T switch in Black Forest, Colorado had failed and cut most of SAC and NORAD’s communications circuits. AT & T had not provided redundant switches, despite have told the US government that it had done so.
General LeMay of SAC thought that spending money on defence was a waste of time since some Russian bombers would always get through. He believed that all the money should be spent on offensive weapons. ‘ If the Soviets launched an attack with 200 bombers and American forces somehow managed to shoot down 90 percent of those planes, the United States would still be hit by at least 20 H-bombs, if not more.’ [Quoted in the excellent Command and Control. by Eric Schlosser 2013]. Being a psychopath he did not realise that saving even a single American city was worthwhile.
LeMay’s arguments did not prevail and the USA began building its defences.