First, it’s important to remember that the areas being cleared were often in really, really bad shape. So the planners of the time thought radical surgery to remove the “blight” was the only way to save the city from further decay.

The growth of American cities in the early 20th century had outstripped the ability of a decentralized capitalist system to provide good housing for low-income workers. (The same was true, if not worse, in rural areas.) Immigrant groups seeking industrial jobs had poured into big cities, especially those we now think of as the Rust Belt. African-Americans moving from the rural South found a different kind of discrimination in cities like Chicago than they’d faced in the South, but racism and economics still concentrated them into dense sections of the city, into districts of old apartment buildings and carved-up 19th century mansions that became some of the nation’s most shameful slums.

The blight spreading in American cities was of great concern to the new field of city planners in the 1930s, as New Deal–financed housing surveys disclosed the severity of the problem. A survey of Chicago buildings completed in 1941 found just under a million dwelling units. More than 200,000 of them were deemed inadequate (poor condition or overcrowded); over 80,000 units lacked private baths or toilets.

After World War II, there were new programs to encourage construction of housing—not just in the suburbs, but in city apartment complexes as well. But what to do about the slum areas? Looking at the building stock, much of it cheaply built decades prior and poorly maintained ever since, planners concluded that the only solution was to clear areas of many blocks at a time, introduce a more modern “superblock” street network that minimized traffic and intersections, and build new residential complexes, shopping centers, and industrial parks. Conservation of the existing buildings seemed impractical, and planners and policymakers were convinced that failing to stop the spread of blight threatened the health of the entire city. Though this was a nationwide movement that made use of federal money, it was the cities that did the land clearance and assembly (sometimes through quasi-independent "authorities"), pushing to save themselves with what seemed like the best approach.

As with many enormous undertakings, there’s many a slip twixt cup and lip. Politics and economics, particularly postwar inflation, threw up unforeseen challenges. Residential areas were cleared without giving adequate attention to where residents would find other housing, and without understanding what that did to both extended family and informal relationships. Often, decades passed before the replacement neighborhoods were ready, and typically at much higher rents.

During those decades, other programs were trying to help the cities. Federal involvement spurred public housing for low-income families, but politicians, reflecting the racial attitudes of their constituents, refused to approve integrated public housing complexes in many areas. The cleared slum areas were often an obvious place to locate them. Similarly, once financing of new freeways came through in 1956, cities found it much easier to use empty urban renewal land than to raze healthy neighborhoods or commercial districts, as I wrote about a few months ago.

By the 1960s, the side effects of such radical surgery on the nation’s cities were becoming better known, and cities such as Chicago began combining urban renewal land clearance with conservation projects that rehabbed the best of a neighborhood’s existing buildings and built new buildings in place of the worst. Very few large land-clearance and assembly efforts took place after 1975, with the exception of ones spurred by specific industrial and commercial users: promised new auto or pharmaceutical plants, downtown shopping or entertainment areas, sport stadiums.