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The peninsula formed by the St. Croix River and the Mississippi River was neutral territory between the Sioux and the Chippewa leaving it relatively open for settlement.
Croix County was created in 1840 from part of Crawford County by an act of the Territorial Legislature when Wisconsin was still a territory.
The St. Croix River was opened to trade and travel in 1839 with the arrival of the steamer Palmyra.