After the Second World War, Switzerland made a deal with the US that the US would provide them with natural resources at an affordable price if Switzerland would oppose the Soviet Union.
The Swiss realized, rightly so, that the Soviets would find out (there were secret agreements between NATO and Switzerland, Sweden, and Austria).
They also realized that in a Third World War, the Soviets wouldn’t even bother to try and invade, but just bomb them with nuclear weapons.
Hence the need for a lot of deep bunkers.
Switzerland had enough supplies to last for 2 years after a nuclear bombardment.
