@CheekyLad_ @kybermace The reason why is that it is so massively effective that it will relieve so many problems that it doesn't just "stop at 1 city"

Because the amount of local resources being absolutely devoured by invaders is so much that if you remove even 10% of them things will improve so quickly that people will go "wow, what would it look like if we got rid of another 10% of these people that absolutely deserve to go"