Let's pretend that somewhere around our current time there exists a secret superweapon that kills humans nearly instantly but doesn't cause significant damage to structures or nature (flora and fauna). And it is used globally. At the same time, there is a secret defense initiative that installed shields in major cities. Let's say, the cities with population of 2+ million people. In Europe these are Istanbul, Moscow, London, St.Petersburg, Berlin, Madrid, Rome, Kiev and Paris.
So, everybody outside these cities die. What will happen with the city folks? Will they die of hunger, because they lose agricultural import from rural areas? Or do they have the capacity to restore the food supply? Or maybe even create food supply within the city? And will they be able to spread their population to the empty lands (there is no radiation, just damaged structures and wilderness) or will they become closed city states?