At the end of the 18th cent. the Illinois, Sac, Fox, and other Native American groups were living in the river forests, where many centuries before them the prehistoric Mound Builders had dwelt. French explorers and missionaries came to the region early. Father Marquette and Louis Jolliet, on their return from a trip down the Mississippi, paddled up the Illinois River in 1673, and two years later Marquette returned to establish a mission in the Illinois country.
In 1679 the French explorer Robert Cavelier, sieur de La Salle, went from Lake Michigan to the Illinois, where he founded (1680) Fort Creve Coeur and with his lieutenant, Henri de Tonti, completed (1682–83) Fort St. Louis on Starved Rock cliff. French occupation of the area was sparse, but the settlements of Cahokia and Kaskaskia achieved a minor importance in the 18th cent., and the area was valued for fur trading.
By the Treaty of Paris of 1763, ending the French and Indian Wars, France ceded all of the Illinois country to Great Britain. However, the British did not take possession until resistance, led by the Ottawa chief, Pontiac, was quelled (1766). In the American Revolution, George Rogers Clark and his expedition captured (1778) the British posts of Cahokia and Kaskaskia before going on to take Vincennes. The Illinois region was an integral part of the Old Northwest that came within U.S. boundaries by the 1783 Treaty of Paris ending the American Revolution. Under the Ordinance of 1787 the area became the Northwest Territory. Made part of Indiana Territory in 1800, Illinois became a separate territory in 1809.
The fur trade was still flourishing throughout most of Illinois when it became a state in 1818, but already settlers were pouring down the Ohio River by flatboat and barge and across the Genesee wagon road. In 1820 the capital was moved from Kaskaskia to Vandalia. The Black Hawk War (1832) practically ended the tenure of the Native Americans in Illinois and drove them W of the Mississippi. In the 1830s there was heavy and uncontrolled land speculation. Mob fury broke out with the murder (1837) of the abolitionist Elijah P. Lovejoy at Alton and in the lynching (1844) of the Mormon leader Joseph Smith and his brother Hyrum at Carthage.
Industrial development came with the opening of an agricultural implements factory by Cyrus H. McCormick at Chicago in 1847 and the building of the railroads in the 1850s. During this period the career of Abraham Lincoln began. In the state legislature, Lincoln and his colleagues from Sangamon co. had worked hard and successfully to bring the capital to Springfield in 1839. As Illinois moved toward a wider role in the country's affairs, Lincoln and another Illinois lawyer, Stephen A. Douglas, won national attention with their debates on the slavery issue in the senatorial race of 1858. In 1861, Lincoln became president and fought to preserve the Union in the face of the South's secession. During the Civil War, Illinois supported the Union, but there was much proslavery sentiment in the southern part of the state.
By the 1860s industry was well established, and many immigrants from Europe had already settled in the state, foreshadowing the influx still to come. Immediately after the Civil War, industry expanded to tremendous proportions, and the Illinois legislature, by setting aside acreage for stockyards, prepared the way for the development of the meatpacking industry. Economic development had outrun the construction of facilities, and Chicago was a mass of flimsy wooden structures when the fire of 1871 destroyed most of the city.
In the latter part of the 19th cent. farmers in the state revolted against exorbitant freight rates, tariff discrimination, and the high price of manufactured goods. Illinois farmers enthusiastically joined the Granger movement. Laborers in factories, railroads, and mines also became restive, and from 1870 to 1900 Illinois was the scene of such violent labor incidents as the Haymarket Square riot of 1886 and the Pullman strike of 1894.
In the 20th cent. labor conditions improved, but violent labor disputes persisted, notably the massacre at Herrin in 1922 during a coal-miners' strike and the bloody riot during a steel strike at Chicago in 1937. State politics became divided by the conflicting forces of farmers, laborers, and corporations, and opposing political machines came into being downstate and upstate.
In 1937 new oil fields were discovered in southern Illinois, further enhancing the state's industrial development. During World War II the nation's first controlled nuclear reaction was accomplished at the Univ. of Chicago, paving the way for development of nuclear weapons during the war. The war also spurred the further growth of the Chicago metropolitan area, and in the postwar period thousands of African Americans from the rural south came seeking industrial work.
Adlai E. Stevenson, governor of Illinois from 1949 to 1953, achieved national prominence in winning the Democratic presidential nomination in 1952 and 1956. Also during the 1950s the "gateway amendment" to the Illinois constitution simplified the state's constitutional amendment process. In 1970, Illinois adopted a new state constitution that, among other reforms, banned discrimination in employment and housing.
Southern Illinois experienced population declines in the 1950s and 60s as farms in the south became more mechanized, providing fewer jobs in the area. The area was hard hit again in the 1980s as farm prices fell and farm machinery, the major industrial product of southern Illinois, was no longer in high demand. The northern portion of the state saw a major decline in manufacturing in the 1970s and 80s, which was partially offset by an increase in the service and trade industry and Chicago's continued strength as a financial center. The election of Ronald Reagan as president in 1980 was the first time someone born in the state had won that office, and Barack Obama, who became the first African-American to be elected president in 2008, had served as U.S. senator from Illinois before his election. Flooding along the Mississippi inundated large areas of W Illinois in 1993.