In the UK the end of the second world war led to increased efforts by planners to shape the direction of cities. Destruction in the Blitz was to many town planners and architects 'a blessing in disguise', allowing them to reconfigure cities in a more rational way. But cities are a good candidate for an example of 'spontaneous order'—that amorphous concept popular with libertarians—where information is dispersed so widely that central plans don't work as well as a common knowledge framework and private direction.
What I hadn't realised until recently was that this process went on extensively even in the USA. The University of Oklahoma's Institute for Quality Communities has a wonderful set of pages comparing aerial photographs of cities 60 years apart—in the 1950s and the early-2010s—and the destruction wrought by planners is evidence even from a cursory glance. The pictures of Midwestern cities are particularly striking.
Kansas City, which in 1955 has the look of a hardboiled-era Los Angeles from above, all consistent blocks, leafy but fairly densely populated, is saddled with an incredible set of highways and junctions right through the middle of the city (I-70 and I-435). Some Spaghetti Junction-esque interchanges required what looks like a dozen blocks to be bulldozed.
In the US, this movement came partly from Harry Truman's Housing Act of 1949 and surrounding bills. This provided huge federal funds ($3trn in today's dollars) to buy up and demolish housing—local government would pay the remainder to get new buildings up. It also came partly from Dwight Eisenhower's Federal Aid Highway Act of 1956, which did a similar thing but for giant roads.
Perhaps the most striking lesson from these illustrations is one that should already be well-learned: density needn't be unattractive. Just as Pimlico and Chelsea are packed with people but among the most desirable places in the world, the 1950s versions of great US cities were denser but prettier. A lot was lost when trying to reconstruct them from the top.