Why are there two Dakotas?

It's a natural question: neither one has that many people, and it's not as if the combined state would be that big, particularly compared to next-door Montana. So why do we have North and South Dakota?

There are a few reasons that the Dakota Territory was split in half before admission into the Union, actually. There were two railroads through the territory: one in northern Dakota leading to Minnesota and Wisconsin, the other in southern Dakota leading to Iowa and Illinois. There was no steel connector between them, only a dirt road, and thus travel was extremely difficult and expensive, the easiest way to do so being a ticket to Chicago and then going back all the way west.

This separation also contributed to the sense of the residents that there were really two regions there, along with the more prominent reason that there were three major settled areas: the Black Hills, in the southwest; the southeast, around the then-capital of the territory, Yankton, and the northeast, around Bismarck, which would be the capital of North Dakota. Lastly, the land usage was quite different, being mostly farming in the north, and grazing in the south. That might not seem like a big deal to city-dwellers (like me), but farmers and ranchers have long had a testy relationship, and avoiding that conflict does seem like it would have been a good argument for the split.

So those were the geographic, sociological, and economic reasons for having a North Dakota and a South Dakota. But naturally, that's not the whole story. See, Dakotans started pressing for a split, and separate admission into the Union, in 1877. For the past few decades, states had been brought into the Union as pairs; before the Civil War, balancing slave states and free states was crucial, and afterwards, neither party wanted to give the other a structural advantage in either house of Congress. But the Dakota Territory was largely populated by Republicans, and from 1875 to 1888, at least one house of Congress was controlled by Democrats, usually the House. Admitting a single Republican state had little appeal to them, and getting two more was even less acceptable.

So for thirteen years Democrats blocked both separation and statehood, while the case grew stronger for the latter as the population increased from 14,000 in 1870 to 539,000 in 1890. People were flocking to Dakota in droves, largely on the strength of the arable land next to the railroads, and when Republicans took the House in 1889, North and South Dakota were admitted promptly, along with Montana, Wyoming, Idaho, and Washington later in the session. All six states sent Republicans to the House and Senate, though since the House was four and a half times as large as the Senate, and there were seven new Republican Representatives to the twelve incoming Republican Senators, the latter was rather more significant.

Most facts courtesy History of the Dakota Territory, Volume 2, by George Washington Kingsbury, published in 1915 by S. J. Clarke Publishing Company.