How did nuclear weapons affect ideas about permanent fortifications after World War II?

Following World War II, most military theorists initially considered traditional permanent fortifications economically impractical due to their vulnerability to nuclear explosives and aerial vertical envelopment. However, this perspective shifted as nations sought to protect strategic assets, leading to the development of specialized, "hardened" defensive works.1

The most prominent example was the intercontinental ballistic missile (ICBM) silo—a tubular, heavily reinforced concrete structure sunk into the ground to protect missiles from blast and shock. These silos became central to strategic calculations, with planners performing elaborate assessments on the number of warheads required to destroy them.1

Beyond missile sites, permanent fortifications were adapted for command and control. A notable example is the North American Air Defense Command (NORAD) complex, which was embedded deep within the rock of Cheyenne Mountain to ensure operational continuity. Other proposed concepts included networks of mobile shelters and "dense pack" basing, which sought to exploit the phenomenon of fratricide to protect hardened silos. Ultimately, these innovations ensured that permanent fortifications remained a vital component of strategic defense throughout the nuclear age.12