The alcohol ban in the 20's was for the entire US and not just Chicago. It was referred to as "Prohibition". It was a constitutional ammendment that started it and another one that ended it.
In hindsight, I am hearing that the real people behind prohibition was the oil industry. Because, up until alcohol was banned, all cars ran off alcohol. Such that, people would simply make it themselves or buy it from a local distillery. When alcohol was banned, cars were converted to gasoline, which was cheap and plentiful back then.
The axiom is that when something bad happens, and you want to know who was really behind it, figure out who had the most to gain, and you'll have the culprit. The same is true for that drug shooting where you live.