What historical event started the corruption?
Don't know for sure.
“What historical event started the corruption?”
I don’t really think any specific historical event could be fingered as the beginning - Chicago machine politics just kind of evolved.
Up until the ‘60’s or so, Chicago was largely a city of ethnic neighborhoods - if you told me your address, I could make a pretty good guess as to whether you were Irish or Polish or Italian or Lithuanian or Black or Mexican, etc. - to a certain extent, you can still do that a little bit today.
The various ethnic groups were able to trade their votes as voting blocs for pieces of political power; over a period of time, trafficing in political influence became more and more the standard for local governance, hence the Democrat machine.
The same thing happened in most other big cities - you had the Tweed gang in New York which evolved into the LaGuardia machine; in Boston you had the Curley machine; I’m sure there were lots of others.
The pols in Chicago were just better at it than most - part of that had to do with the original Mayor Daley, who tolerated incredible corruption, but lived in a bungalow in his modest neighborhood of Bridgeport (which in fact was corruption central for the machine). The first Daley kept the city running relatively efficiently, but the corruption was so firmly embedded that once he kicked off, the whole racket got worse, but city services pretty much began to deteriorate as well.
To give you an idea how screwed up the city is - it’s got just about the worst school system in country, neighborhood shootings are an almost daily occurence, the transit system is in a constant state of near-collapse, streets are pot-hole ridden — and every last one of the 50 alderman in the city council is a democrat, they all have been for the last several decades, and they all will continue to be as far as the eye can see.
But at least we’re not Detroit.