"Treaties—solemn agreements between sovereign nations—lie at the heart of the relationship between Indian Nations and the United States. Native Nations made treaties with one another long before Europeans came to the Western Hemisphere. The United States began making treaties with Native Peoples because they were independent nations. Often broken, sometimes coerced, treaties still define mutual obligations between the United States and Indian Nations."
Excerpt from Smithsonian Institution, National Museum of the American Indian, "Nation to Nation: Treaties Between the United States and American Indian Nations."
An excellent resource for looking up treaties and cessions is digitreaties.org, created by the Museum of Indian Arts and Culture (Santa Fe, New Mexico), in partnership with the US National Archives Office of Innovation and National Archives Foundation.
Six different treaties between 1795 and 1833 forced the land from Native peoples to the U.S. government:
Map by Dennis McClendon
1795 Treaty of greenville
Treaty Between the United States and the Wyandot, Delaware, Shawnee, Ottawa, Chippewa, Potawatomi, Eel River, Wea, Kickapoo, Piankashaw and Kaskaskia Indians Signed at Greenville (1795)
This treaty ceded most of what is now Ohio as well as 16 smaller plots of land at strategic locations on waterways, including three in present-day Illinois:
A six-mile square piece of land at the mouth of the Chicago to build Fort Dearborn
The portage between the Chicago and Des Plaines Rivers
The mouth of the Illinois River.
Full text from Kappler's Indian Affairs: Laws and Treaties (1904)
Most debates about ownership of Lake Michigan center on the public trust doctrine, put forth in the landmark 1892 U.S. Supreme Court case of Illinois Central Railroad Co. v. Illinois. The state of Illinois sued Illinois Central Railroad for who had the right to build out into the lakebed by the land owned by the railroad along Chicago’s lakefront. The case affirmed that each state holds title to the submerged lands within its territory, and that those lands are held in trust by the state for use by the people for commercial and recreational use.
Title to the bed of Lake Michigan had been conveyed from the federal government to Illinois upon statehood in 1818. However, the treaties ceding land in Illinois were negotiated by the federal government in 1833. Although the federal government had jurisdiction to take possession of the land itself, title to the lakebed belonged to the state. In fact, the State of Illinois should have negotiated with Native peoples for the lakebed, which it never did.