What if the people in your capital cities cannot eat?
Nations are usually governed from a single capital city, so for strategic reasons, it is important that the people in the capital city have food to eat. However, as people have moved from rural to urban areas, and as (some) urban areas have dramatically increased in population, the ability of some capital cities to be self-sustaining has declined.
As a University of Copenhagen study notes:
Higher farmland yields have influenced the cities self-provisioning over the past 40 years, but overall the ability of cities to feed themselves is unlikely to keep pace with increasing population, the research shows….
Particularly in the capitals of Australia and Japan, where the population has increased tremendously over the past 40 years, the self-provision has declined; in Canberra from 150 to 90 percent and in Tokyo from 41 to 27 percent.
This is despite the increase in yield of agricultural land per hectare. Copenhagen on the other hand, has increased its self-provision slightly from 34 to 45 percent because its population has remained fairly constant.
More information about the study, and its implications, can be found here.