The answer(s)
2018-01-18: No, it was not !

2019-04-18: It was an important city at that time, but the first American city was thousands of years older. Evidence of these cities are yet to discover.

The story
The Cahokia Mounds State Historic Site is the site of a pre-Columbian Native American city (circa 1050–1350) directly across the Mississippi River from modern St. Louis, Missouri. This historic park lies in southern Illinois between East St. Louis and Collinsville. The park covers 2,200 acres (890 ha), or about 3.5 square miles (9 km2), and contains about 80 mounds, but the ancient city was much larger. In its heyday, Cahokia covered about 6 square miles (16 km2) and included about 120 manmade earthen mounds in a wide range of sizes, shapes, and functions. Cahokia was the largest and most influential urban settlement of the Mississippian culture, which developed advanced societies across much of what is now the central and southeastern United States, beginning more than 1,000 years before European contact. Today, Cahokia Mounds is considered the largest and most complex archaeological site north of the great pre-Columbian cities in Mexico.

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