Native Americans lived along the waterways of the Illinois area for thousands of years before the arrival of Europeans. The Koster Site has been excavated and demonstrated 7,000 years of continuous habitation. Cahokia, the largest regional chiefdom and urban center of the Pre-Columbian Mississippian culture, was located near present-day Collinsville, Illinois. They built more than 100 mounds and a woodhenge in a planned design expressing the culture's cosmology. The civilization vanished in the 15th century for unknown reasons, but historians and archeologists have speculated that the people depleted the area of resources. Many indigenous tribes engaged in constant warfare. According to Suzanne Austin Alchon, "At one site in the central Illinois River valley, one-third of all adults died as a result of violent injuries."
The next major power in the region was the Illinois Confederation or Illini, a political alliance among several tribes. There were about 25,000 Illinois Indians in 1700, but systematic attacks and warfare by the Iroquois reduced their numbers by 90%. Gradually, members of the Potawatomi, Miami, Sauk, and other tribes came in from the east and north. In the American Revolution, the Illinois and Potawatomi supported the American colonists' cause.
French explorers Jacques Marquette and Louis Jolliet explored the Illinois River in 1673. In 1680, other French explorers constructed a fort at the site of present day Peoria, and in 1682, a fort atop Starved Rock in today's Starved Rock State Park. As a result of this French exploration, Illinois was part of the French empire until 1763, when it passed to the British with their conquest of New France. The small French settlements continued; a few British soldiers were posted in Illinois, but there were no British or American settlers. In 1778, George Rogers Clark claimed the Illinois Country for Virginia. The area was ceded by Virginia to the new United States in 1783 and became part of the Northwest Territory.
The Illinois-Wabash Company was an early claimant to much of Illinois. The Illinois Territory was created on February 3, 1809, with its capital at Kaskaskia.
During the discussions leading up to Illinois' admission to the Union, the proposed northern boundary of the state was moved twice.The original provisions of the Northwest Ordinance had specified a boundary that would have been tangent to the southern tip of Lake Michigan. Such a boundary would have actually left Illinois with no shoreline on Lake Michigan at all. However, as Indiana had successfully been granted a 10-mile northern extension of its boundary to provide it with a usable lakefront, the original bill for Illinois statehood, submitted to Congress on January 23, 1818, stipulated a northern border at the same latitude as Indiana's which is defined as 10 miles (16 km) north of the southernmost extremity of Lake Michigan. But the Illinois delegate, Nathaniel Pope, wanted more. Pope lobbied to have the boundary moved further north, and the final bill passed by Congress included an amendment to shift the border to 42° 30' north, which is approximately 51 miles (82 km) north of the Indiana northern border. This shift added 8,500 square miles (22,000 km2) to the state, including the lead mining region near Galena. More importantly, it added nearly 50 miles of Lake Michigan shoreline and the Chicago River. Pope and others envisioned a canal which would connect the Chicago and Illinois rivers, and thusly, connect the Great Lakes to the Mississippi.
In 1818, Illinois became the 21st U.S. state. The capital remained at Kaskaskia, headquartered in a small building rented by the state. In 1819, Vandalia became the capital, and in the next 18 years, three separate buildings were built to serve successively as the capitol building. In 1837, the state legislators representing Sangamon County, under the leadership of state representative Abraham Lincoln, succeeded in having the capital moved to Springfield, where a fifth capitol building was constructed. A sixth capitol building was erected in 1867, which continues to serve as the Illinois capitol today.
Though ostensibly a "free state", Illinois had slavery. The French owned black slaves as late as the 1820s. Slavery was nominally banned by the Northwest Ordnance, but that was not enforced. When Illinois became a sovereign state in 1818, the Ordnance no longer applied, and there were about 900 slaves there. As the southern part of the state, known as "Egypt", was largely settled by migrants from the South, the section was hostile to free blacks and allowed settlers to bring slaves with them for labor. Most citizens were opposed to allowing blacks as permanent residents, and efforts to make slavery official failed in 1822. Nevertheless, some slaves were brought in seasonally or as house servants. The Illinois Constitution of 1848 was written with a provision for exclusionary laws to be passed. In 1853, John A. Logan helped pass a law to prohibit all African Americans, including freedmen, from settling in the state.
In 1832, the Black Hawk War was fought in Illinois and current day Wisconsin between the United States and the Sauk, Fox and Kickapoo Indian tribes. The Indians withdrew toIowa; when they attempted to return, they were defeated by U.S. militia and forced back to Iowa.
The winter of 1830–1831 is called the "Winter of the Deep Snow"; a sudden, deep snowfall blanketed the state, making travel impossible for the rest of the winter, and many travelers perished. Several severe winters followed, including the "Winter of the Sudden Freeze". On December 20, 1836, a fast-moving cold front passed through, freezing puddles in minutes and killing many travelers who could not reach shelter. The adverse weather resulted in crop failures in the northern part of the state. The southern part of the state shipped food north and this may have contributed to its name: "Little Egypt", after the Biblical story of Joseph in Egypt supplying grain to his brothers.
By 1839, the Mormon utopian city of Nauvoo, located on the Mississippi River, was created, settled, and flourished. In 1844, the Mormon leader Joseph Smith was murdered in the Carthage jail. After close to six years of rapid development, the Mormon city of Nauvoo, which rivaled Chicago as Illinois' largest city, saw a rapid decline after the Mormons left Illinois in 1846 for the West in a mass exodus.
Chicago gained prominence as a Great Lakes port and then as an Illinois and Michigan Canal port after 1848, and as a rail hub soon afterward. By 1857, Chicago was Illinois' largest city. With the tremendous growth of mines and factories in the state in the 19th century, Illinois played an important role in the formation of labor unions in the United States. The Pullman Strike and Haymarket Riot in particular greatly influenced the development of the American labor movement. From Sunday, October 8, 1871, until Tuesday, October 10, 1871, the Great Chicago Fire burned in downtown Chicago, destroying 4 square miles (10 km2).
In 1847, after lobbying by Dorothea L. Dix, Illinois became one of the first states to establish a system of state-supported treatment of mental illness and disabilities, replacing local almshouses.
During the American Civil War, over 250,000 Illinois men served in the Union Army, a figure surpassed by only New York, Pennsylvania, and Ohio. Beginning with President Abraham Lincoln's first call for troops and continuing throughout the war, Illinois mustered 150 infantry regiments, which were numbered from the 7th to the 156th regiments. Seventeen cavalry regiments were also gathered, as well as two light artillery regiments.
In the 20th century, Illinois emerged as one of the most important states in the union, with a population of nearly 5 million bolstered by continued immigration from southern and eastern Europe, and by African-Americans from Mississippi, Louisiana, and Arkansas. By the end of the century, the population had reached 12.4 million. The Century of Progress World's Fair was held at Chicago in 1933. Oil strikes in Marion County and Crawford County lead to a boom in 1937, and, by 1939, Illinois ranked fourth in U.S. oil production.
No state has had a more prominent role than Illinois in the emergence of the nuclear age. As part of the Manhattan Project, the first sustained nuclear chain reaction took place at the University of Chicago in 1942. In 1957, Argonne National Laboratory, near Chicago, activated the first experimental nuclear power generating system in the United States. By 1960, the first privately financed nuclear plant in United States, Dresden 1, was dedicated near Morris. In 1967, Fermilab, a national nuclear research facility near Batavia, opened a particle accelerator which was the world largest for over forty years. And, with eleven plants currently operating, Illinois leads all states in the amount of electricity generated from nuclear power.
Chicago became an ocean port with the opening of the Saint Lawrence Seaway in 1959. The seaway and the Illinois Waterway connected Chicago to both the Mississippi River and the Atlantic Ocean. In 1960, Ray Kroc opened the first McDonald's franchise in Des Plaines (which still exists today as a museum, with a working McDonald's across the street).
The state's fourth constitution was adopted in 1970, replacing the 1870 document. The first Farm Aid concert was held in Champaign to benefit American farmers, in 1985. The worst upper Mississippi River flood of the century, the Great Flood of 1993, inundated many towns and thousands of acres of farmland.