In October 14th, 1951 President Grace ordered an emergency evacuation of Chicago. Unfortunately, the evacuation came too late to save the majority of the civilian population as the Chimera launched a massive spire attack that infected most of the city within minutes, and all the surrounding areas in a 50 mile radius of downtown Chicago. The city became America's second Gray Zone two months after the Chimeran attacks in New England. Sometime during the Chimeran occupation, a Chimeran tower (dubbed the Chicago Tower) under Chicago was excavated.
Within months Chimeran forces were amassing in the city, posing a threat to the Liberty Defense Perimeter. It is not until SRPA initiated Operation Shadow Strike on April 19th, 1952 in which Specter Team successfully decimated and contained the Chimeran threat in Chicago by April 22nd. It is during this operation that SRPA recon Team X-Ray infiltrate the Chicago Tower and learned that it was linked to seven other towers; and also important in the tower is Specter's discovery of ancient, advanced artifacts known as "Gray Tech", which the team itself is eventually tasked to find and recover these artifacts.
After the events of Operation Shadow Strike, Specter Team and intelligence analysts at SRPA 3 had reported that the Chimera constructed a 300-foot-tall enclosure surrounding Wrigley Field which SRPA ignored the enclosure as unimportant. Unfortunately, SRPA failed to realize that it is this facility that soon created the Leviathan. By the time of the Chimeran invasion most of the city's street and surroundings are submerged in Fury-infested waters. Furthermore, despite the city's occupation, there are a few handful of several hundreds of human survivors who decided to remain in the city and establish a resistance front against the Chimera.
Within a year later, Daedalus launched a massive assault on America and the Chimera restarted the Chicago Tower. The Sentinels and SRPA forces returned to Chicago to shut down the tower. After defeating the city's Chimeran forces and along with the Leviathan, SRPA manage to gain total access to the tower as well in apparently regaining control of Chicago. However, Dr. Fyodor Malikov was unable to shut down the tower after it was remotely reactivated by Daedalus from Iceland.