Chicago was a pre-war city in the United States of America. The largest city in the Midwest Commonwealth, and second largest in the country, Chicago was a major industrial hub (Particularly for the American railroad industry). Beneath the clandestine surface, however, Chicago was almost completely controlled by the mob for most of the 21st century, and even after the bombs fell, the Chicago "families" were still running the city from the safety of their vaults.
The first major change to the city after the Fallout universe diverged from our own was the redesign of the Chicago flag. In 1947, a fifth star was added in honor of America's entry into the atomic age. In the mid-1950s, the Chicago Outfit (The south side mob) began to expand its power within the city. By the mid 21st century, the mob had more control over the city than the mayor’s office. The final straw occurred in 2061, when the mayor of Chicago (A short and rather insecure fellow) bankrupted the city by constructing “The Illinois”, a massive mile-high skyscraper designed by Frank Lloyd Wright, in a misguided attempt to earn his city some recognition. The leader of the Chicago Outfit (Rumored to be descended from the infamous Al Capone) won the mayoral election in a landslide, effectively turning the entire windy city into mob territory. While the energy crisis of the 2060s ravaged the rest of the world, Chicago managed to stay relatively stable (Due to an illegal petroleum smuggling racket) and the mob reigned supreme until that fateful day, October 23, 2077.
The mayor and his associates had donated generous sums to Vault-Tec in order to guarantee themselves and their families sanctuary in their vaults, and as a result, the Chicago outfit was not destroyed along with the city. The Chinese atom bombs completely ravaged the metropolis, decimating most of the skyscrapers (Such as “The Illinois”, which was now reduced to its ground-level floor) save for the Sears Tower which had managed to miraculously survive the blasts thanks to a combination of SCIENCE! and sturdy construction. Afterward, the entire region was plunged into a bitter, perpetual state of winter.
A few years after the mayor died (at the ripe old age of 93) below the ground, his three top associates (Known only by their last names of O’Leary, Pulaski and Belushi) emerged from their vaults almost three decades after the bombs fell. The three families (Irish mob, Polish mob and Italian mob, respectively) soon found themselves engaged in a bloody war over control of the burnt-out, snow-covered ruins. Eventually, after years of fighting, borders were drawn, and the city was divided into three districts: The “north side” (run by the Italians), the “south side” (run by the Irish) and the “loop territories” (run by the Poles). In the 100 or so years afterward, the three districts have experienced an uneasy peace, and have attempted to rebuild their sections of the city (This is not to say that occasional skirmishes between the families were unheard of). With the eventual repair of some nearby power-stations, the wealthier citizens are privy to conveniences such as electricity and running water, while streetcars and elevated trains operate amongst the Brahmin carriages and rooftops, respectively. The red aircraft warning light on top of the 232-year-old Sears Towers’ roof still functions every night, drawing in various travelers from all over the midwest, curious to find the source of the “mysterious blinking red light in the sky”…