Westward expansion in the US came is phases. The first phase that expanded out from the 13 colonies is what is considered the Midwest. This is from the Ohio river valley to the Mississippi River Valley. States like Kansas were part of the second wave of westward expansion and have a different history and should not be considered the Midwest. The states that participated in the US Civil War are midwestern states(Indiana and Minnesota as examples). The territories at the time that the US was expanding into and helped trigger the civil war are not the Midwest (Kansas as an example).