On both sides of the Mississippi, between the mouths of the Wisconsin and the Rock, lie the extensive lead fields which attracted Du Buque in the days of the Spanish rule, and which now in the twenties induced an American immigration. The ease with which these diggings could be worked and the demand of a growing frontier population for lead, brought miners into the borderland of Illinois, Wisconsin, and Iowa long before either of the last states had acquired name or boundary or the Indian possessors of the soil had been satisfied and removed. The nations of Winnebago, Sauk and Foxes, and Potawatomi were most interested in this new white invasion, while all were reluctant to yield the lands to the incoming pioneers. The Sauk and Foxes had given up their claim to nearly all the lead country in 1804; the Potawatomi ceded portions of it in 1829; and the Winnebago in the same year made agreements covering the mines within the present state of Wisconsin. Gradually in the later twenties the pioneer miners came in, one by one. From St. Louis they came up the great river, or from Lake Michigan they crossed the old portage of the Fox and Wisconsin. The southern reënforcements looked much to Fort Armstrong on Rock Island for protection. The northern, after they had left Fort Howard at Green Bay, were out of touch until they arrived near the old trading post at Prairie du Chien. War with the Winnebago in 1827 was followed in 1828 by the erection of another United States fort,--at the portage, and known as Fort Winnebago. Thus the United States built forts to defend a colonization which it prohibited by law and treaty.