[jjk166]

That was always a bit of a myth. Walls are not meant to stop things from getting destroyed, they are to prevent easy entry. Even in antiquity the ability to lob something over a wall existed, but if you were in range to do so, the defenders on the walls were in range to lob things at you, structures within the walls could be hardened to resist damage from things lobbed at them, and ultimately lobbing things over walls simply didn't win conflicts. Even the strategic bombing of WW2 wasn't sufficient to bring any side to its knees - only boots on the ground or the imminent threat thereof actually got the job done.

Further, the disappearance of walls does not coincide well with developments in weapons technology. Walls adapted to the introduction of gunpowder and explosives, with fortresses being key parts of strategy well into the 20th century. Even medieval fortifications with minimal upgrades still proved reasonably effective in modern conflicts. Walls are very good for their intended purpose.

The disappearance of city walls was not due to technological but rather social progress. The early modern period saw the development of strong central states able to field large armies. These states did not want a lot of fortified cities that could close themselves off easily from either a foreign invader or from their own government. Instead the national army would defend cities as needed, operating from fortresses in strategic locations and setting up temporary fortifications as needed. Cities were redesigned to make it easy to march an army into them. At the same time, population growth and changing economic systems meant cities rapidly grew, far outstripping the limited space available within medieval walls. Again, detached networks of mutually-supporting forts were simply more economical than contiguous walls. Finally, the changing world meant that you were simply no longer worried about wandering war bands pillaging settlements. Most of what walls were needed for could be done more economically with fences and legal markers.

[IAmBroom]

> Even the strategic bombing of WW2 wasn't sufficient to bring any side to its knees - only boots on the ground or the imminent threat thereof actually got the job done.

Ironic, given that two strategic bombs did in fact end WW2. There were imminent threats, Normandy-style, anticipated, and the Japanese were prepared to fight it to the death. It is widely agreed that a Fat Man and a Little Boy turned the war.