Historically, cities have depended upon rural production - food surpluses produced in the countryside. In order to purchase food surpluses, cities themselves produced things that rural folks desired - tools, entertainment, and marketplaces for the exchange of rural produce. Cities became places of exchange - of people and ideas and products. And they're still that way. The city dies.
Technology revolutionizes geography.
The typical international airline flight now costs about a sixth of what it did in 1940. A phone call to Tokyo costs one-sixtieth of what it did in 1930. You can send an overseas cable for something like a tenth of the 1970 cost, and one-thousandth of the 1860s cost. All of this makes it much easier for private citizens to find out what's going on around the world. This is where we come to economy.