At the end of the 1950s, reflecting both the threat of possible invasion by the Soviet Union and a nuclear warfare, Swiss military doctrine changed to mobile defence that included missions outside its territory, in order to defeat stand-off attacks and nuclear threats, including the possibility of defensive employment of air-delivered nuclear weapons. However the inability to field a force capability to carry out such missions led to the return of traditional "protection of own territory" doctrine. Meanwhile, the army started preparing ad hoc airbases in the mountains, with sections of highway strengthened to act as runways and hangars carved out of the mountains.