1,000 years ago, Illinois had the largest city in what would become the U.S.

This is an aerial view of the 100-foot-high Monks Mound at Cahokia Mounds State Historic Site. Courtesy of Cahokia Mounds State Historic Site

Centuries ago, Illinois was home to the largest and most influential city in what would become the United States, rivaling the size of European cities at the time.

As many as 20,000 people ­­-- double that if surrounding communities were included -- lived about a thousand years ago in the elaborate planned city that now is part of the Cahokia Mounds State Historic Site.

Cahokia, east of St. Louis, includes woodhenges -- large sun calendars that were built of red cedar posts -- and at least 120 mounds and pyramids used to support important buildings and for burials.

Yet Cahokia is only part of Illinois' rich American Indian heritage.

Although 101,451 Illinoisans identify themselves as being fully or partly of American Indian descent, the tribes themselves are gone from Illinois, mostly moved west by the federal government in the 1800s, said Bill Iseminger, assistant manager of the Cahokia Mounds State Historic Site. What's left are the remains of native cemeteries, villages, cities and mounds, onetime Indian routes that became the basis for many of our modern roads and highways, and the names of many things, beginning with our state itself, named for a powerful American Indian group that once called Illinois home.

This is an artist's depiction of the central plaza in a city of 20,000 or more people that flourished 1,000 years ago at what is now the Cahokia Mounds State Historic Site. - Cahokia Mounds State Historic Site/L.K. Townsend

Archaeologists estimate people came here about 12,000 years ago after a migration from Asia over a land bridge began to populate the continent, according to the Illinois State Museum. Over thousands of years, people adapted to a changing climate and became less nomadic as the Ice Age came to a close.

Cahokia was built during the Mississippian Age, which began around 900 A.D. The city was the political and religious center of Mississippian life, but eventually its influence faded. It was largely abandoned by 1350 A.D., though no one knows precisely why.

Wood, game and other food sources might have been depleted, Iseminger said. There is evidence of long droughts, and the climate was cooling.

Where did they go? Probably in many directions, joining other communities or establishing new ones, Iseminger said. The Osage, Ponca, Omaha and Quapaw people are believed to be descended from the city's builders.

About 700 years ago, as Cahokia was fading, a people called the Oneota emerged in the Illinois River valley. Excavations in Fulton County show half of those buried in an Oneota cemetery died as a result of violence, though it's not known why, according to the Illinois State Museum.

Were the two cultures at war? It's hard to say, though there's evidence of Oneota culture in the Cahokia area. And Cahokia had a large palisade enclosure, indicating a need for defense, Iseminger said.

"The Oneota, as they migrated south, may have been the final blow of the Mississippian culture," he said.

This is an artist's overhead view of the large and influential city that existed 1,000 years ago at what is now the Cahokia Mounds State Historic Site. - Cahokia Mounds State Historic Site/ William R. Iseminger

By the time French traders and priests arrived in the 1600s, there were two main American Indian groups in the area: the Illiniwek, or Illinois, and the Miami. In the late 1700s and early 1800s, the Illiniwek territory shrank, the Miami moved east, and other tribes moved in.

As Europeans and other outsiders arrived, the Indians traded with them. They allied with the French and the British, at different times. American Indian federations and tribes fought each other, with the Iroquois being notorious for battles against the Illiniwek.

Starved Rock State Park near Utica is named for a stone formation where, legend has it, members of the Illiniwek starved to death in 1769 during a siege by the Ottawa and Potawatomi, who sought to avenge the killing of the Ottawa chief Pontiac.

By 1818, a War Department study estimated there were at least 10,860 native people living in this area, not counting the Peoria band of Miami Indians. Tribes had already ceded a strip of land at the mouth of the Chicago River to the U.S. government, as well as land near Kaskaskia.

In 1821, the tribes gave up more of northern Illinois.

In 1832, some members of the Sac and Fox nations agreed with a warrior named Black Hawk that their land had been illegally taken from them. They tried moving back into northwestern Illinois and western Wisconsin, leading to the unsuccessful Black Hawk War.

The 1833 Treaty of Chicago took care of the largest swath. In exchange for handing over 5 million acres, tribes including the Potawatomi, Ojibwe, Odawa, Sac and Fox agreed to move west of the Mississippi River, according to a 1918 article about the treaty in the Wisconsin Magazine of History. Many would eventually be pushed farther, which is why the Peoria Tribe of Miami Indians is now headquartered in Oklahoma.

- Steve Lundy | Staff Photographer This is an illustration of settlers trading with American Indians from Elijah Haines' 1877 book, "The Past and Present of Lake County, Illinois."

When the French and other Europeans arrived, they learned the tribes' languages and began naming places and landmarks using the natives' words. French or English versions of the names stuck.

The settlers also might have been influenced by the Romantic movement of the late 1700s and early 1800s, which placed a high value on nature and folk legends, so the names might have been meant to memorialize the dwindling numbers of natives.

Poet Walt Whitman extolled the practice in "An American Primer": "All aboriginal names sound good. I was asking for something savage and luxuriant, and behold, here are the aboriginal names. I see how they are being preserved. They are honest words -- they give the true length, breadth, depth. They all fit. Mississippi! -- the word winds with chutes -- it rolls a stream three thousand miles long. Ohio, Connecticut, Ottawa, Monongahela, all fit."

There are six counties in Illinois that have Indian-inspired names, at least 60 towns, and scores of schools and other places, from Aptakisic Road in Lake County to the town of Wetaug in Union County.

Many other roads now have more prosaic names but trace the paths tribes once used, from the east-west Route 6, which generally follows the onetime Sauk Trail, to the North Shore's Green Bay Road, thought to have been a Potawatomi trail.

• Illinois 200 is produced as a project of the Illinois Press Association and the Illinois Associated Press Media Editors.