With the decrease in fertility in the last few decades, the age structure of the population has moved towards old age, which can be considered a threat to national security. The current research aims to examine the changes in fertility and the age structure of the population in the cities of Iran. Statistical data has been collected from the yearbooks of the Iranian Statistics Center and analyzed using spatial statistical methods and GIS. The results show that the decrease in fertility has caused significant changes in the age structure of the population. Various factors such as employment rate, education, and urbanization have a negative relationship with fertility. Also, the analysis shows that there is a negative relationship between fertility and aging. The concentration of aging in the interior and northern cities shows serious challenges in the field of security and defense, which require special attention. The continuation of such a trend will face the country with serious challenges in the field of security and defense, which are in four dimensions: economic, social, political and military was dealt with.