The State of Illinois is a state of the United States, the 21st to be admitted to the Union. It is the most populous and demographically diverse Midwestern state and the fifth most populous state in the nation.
With a population near 40,000 between 1300 and 1400 AD, the Mississippian-culture city of Cahokia, in what is now southern Illinois, was the largest city within the future United States until after 1790, when it was surpassed by New York City. Gradually Cahokia and the area were abandoned, and at the time of the American Revolution, only about 2,000 Native American hunters and a small number of villagers of French ancestry inhabited the Illinois area. United States migrant settlers began arriving from Kentucky in the 1810s; Illinois achieved statehood on December 3, 1818. The future metropolis of Chicago was founded in the 1830s on the banks of the Chicago River, one of the few natural harbors on southern Lake Michigan. Railroads and John Deere's invention of the self-scouring steel plow made central Illinois' rich prairie into some of the world's most productive and valuable farmlands.
While "The Iron Elephant" clearly takes place in Illinois, the story never tells us what the region's name is in the ADF universe. But we know it isn't "Illinois," as this is a timeline where Native Americans' languages and place names never existed.