But for the North Koreans, living in fear of B-29 attacks for nearly three years, including the possibility of atomic bombs, the American air war left a deep and lasting impression. The DPRK government never forgot the lesson of North Korea's vulnerability to American air attack, and for half a century after the Armistice continued to strengthen anti-aircraft defenses, build underground installations, and eventually develop nuclear weapons to ensure that North Korea would not find itself in such a position again. ... The war against the United States, more than any other single factor, gave North Koreans a collective sense of anxiety and fear of outside threats that would continue long after the war's end.[1]