The idea of a CSA-friendly Northwest Confederacy is of course not the same thing as that of a northern state joining the CSA but is almost equally unrealistic. I had an old soc.history.what-post on it at
https://soc.history.what-if.narkive.com/c4pjYslL/what-would-it-take#post7
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And then there was also talk during the ACW of a Northwest Confederacy ("Northwest" here referring to the Old Northwest) that would make a separate peace with the Southern Confederacy:
"In Butternut regions of the Midwest, economic grievances reinforced the cultural attitudes of people descended from southern settlers. The war had cut off their normal trade routes along the Mississippi and its tributaries, forcing them into dependence on Yankee railroads and canals feeding an east-west pattern of trade. Real and imaginary grievances against high rates and poor service on these routes exacerbated the hostility of Butternuts toward New Englanders whom they charged with controlling their destiny through manipulation of Congress as well as the economy. 'Shall we sink down as serfs to the heartless, speculative Yankees,' asked an Ohio editor, 'swindled by his tariffs, robbed by his taxes, skinned by his railroad monopolies?'
"This sense of Butternut identity with the South and hostility to the Northeast gave rise to talk among western Democrats of a 'Northwest Confederacy' that would reconstruct a Union with the South, leaving New England out in the cold until she confessed the error of her ways and humbly petitioned for readmission. However bizarre such a scheme appears in retrospect, it commanded much rhetorical support during the war. 'The people of the West demand peace, and they begin to more than suspect that New England is in the way,' warned Vallandigham in January 1863. 'If you in the East, who have found this war against the South, and for the negro, gratifying to your hate or profitable to your purse, will continue it. . . [be prepared for] *eternal divorce between the West and the East.*' Though less extreme than Vallandigham, Congressman Samuel S. Cox of Ohio agreed that 'the erection of the states watered by the Mississippi and its tributaries into an independent Republic is the talk of every other western man.' This threat to reopen the Mississippi by a separate peace generated General McClernand's proposal to reopen it with his separate campaign against Vicksburg. The whole issue lent an urgency to Grant's efforts to capture Vicksburg and a bitter edge to criticism of his initial failures to do so." James M. Mac Pherson, *Battle Cry of Freedom: The Civil War Era*, p. 593.
http://books.google.com/books?id=-u...n51&sig=_sy0BupY-iL1L5zG-m9m0H6ckhg#PPA593,M1
For one Southern formulation of this idea, see the article "A Northwest Confederacy" in
http://books.google.com/books?id=jqYKAAAAIAAJ&pg=RA2-PA59
"[A letter] by a southern planter, published in the Chattanooga Daily Rebel, attracted considerable attention. It outlined the plan of those who would nurture a new secession in the United States. The document is here reprinted entire as a type of the point of view of those in the south who were looking to the Northwest with hope.
"Hon. H. S. Foote, Richmond:
"DEAR SIR:- Your efforts to suggest some plan by which the war might be shortened have been praiseworthy. So little had been indicated north of the Ohio river that it left every move open to serious objections.
"Time and efforts produce by the valor of our troops, seem to have given existence to a sentiment which deserves a watchful attention from statesmen of the South.
"This sentiment is found among the agricultural interests in the 'Northern' Valley of the Mississippi river, and mainly among the old Democrats of that region. We occupy a position now, and have always done so, that we could not make proposals to the Lincoln Government. That is the true position still. To that, we bid defiance; but to the legislatures of Indiana and Illinois, and other states of the Northern Valley of the Mississippi, which may come to their conclusions, I hold a different policy to be correct. We should meet their resolutions with all the concessions which we can consistently make in trade and general commerce, including, of course, the free navigation of the Mississippi river, upon conditions thus:
"1st. Indiana, Illinois, Wisconsin, Iowa, Kansas and Minnesota, and perhaps Ohio, shall form the 'Northern Confederacy'.
"2d. Both Confederacies, 'Southern' and 'Northern', to be politically independent. All the slave states to belong to the 'Southern onfederacy.'
"3d. A league between the two, offensive and defensive, and runaway slaves to be returned; the navigation of the Mississippi river and free trade, and 'imports' at our tariff.
"Advantages to both 'Confederacies,' The 'Southern Confederacy' obtains peace. A strong Ally in War and protection to slavery. Her independence acknowledged by the Northern Confederacy, which will be be sufficient. She obtains for her seabord cities the importations for both Confederacies, and their freight on her rivers and railroads.
"The 'Northern Confederacy' gets rid of the responsibility of slavery. It may assume whatever portion of the immense war debt now existing, they may decide upon.
"It secures importations at our low tariff. It secures its former marketin the South for its agricultural productions and the same use of the great Mississippi river. Its political independence gives position and place to its rising statesmen. Its topography and unity of pursuit, institutions and labor; secure harmony and legislation, and promise great prosperity. The two together secure the adjacent territories, a very important point; as they cannot be united to the remnant of the old United States, lying East; including New England which brought on the war. The two Confederacies would become the great 'powers' of the American Continent.
"The 'Southern Confederacy,' based upon slave labor, would always preponderate in intellect; and would control.
"I present this to your well stored, prolific mind, as an outline of what may come out of a wise course in eeting the sentiment of the 'North West', heretobefore alluded to.
"It is true they have fought us; invaded our country; and wronged us terribly; but that is done,and cannot be recalled. It is a matter of incalculable advantage to our Confederacy--to stop the sacrifice of life, and of some importance to limit the debt, and restore our citizens to their homes. Concession can be made to the 'Northern Confederacy' formed of the States named; which will stop the war and will benefit us at the same time. As a cotton planter, and slave owner, I would greatly prefer the league, on the terms mentioned, to separate independence, with the enemy of that people, to the institution of slavery. As soon as they are disconnected from slavery, it will cease to be discussed, everywhere.
"If we are not strong, it may generate another war. The League gives great strength. Under this league, can be embraced what they mean by reconstruction. That is, their position will be as good, or better than before.and 22 States will be in the league instead of 33. But the New England States, New Jersey, Delaware, &c., are of no importance to them. They have secured the market and trade, and for these they were fighting; and are also politically disconnected from slavery. Indeed they thus obtained all they are contending for. They say they are not fighting to free the slaves. We obtain all we are contending for.
"I find ultra men, unwilling to do anything, but fight on. They are not in the army, I have been with the army since its organization. I know the opinion and sentiment of the army. They have suffered sufficiently, and desire peace.
"If the North-west are met on the basis proposed herein, I think we will enter the wedge which will sunder the present authorities conducting the war. Lincoln will carry on the war during his administration, if he can get the support of these states. We then should be on the alert, and if possible, deprive him of this portion of his army. The balance we can whip, very soon, if necessary. We can conquer a peace from them; but that will not be necessary. If Indiana and Illinois withdraw the war will close. With these proposals before them, they won't fight longer. The other States named will follow, or some of them at least.
"If this be neglected on our part, the leading men may be offered positions, which would neutralize their efforts.
"These States are a part of the Mississippi Valley and their true alliance is with the South. They are an agricultural people, and so are we; but their products are different from ours, and hence the advantages in a commercial league.
"Negotiations must begin sometime--fighting alone won't adjust a difficulty.
"I have seen so little of the proceedings of Congress, that I am ignorant of what has been discussed.
"The prominent idea is this. We make no proposition to the Government, but we should put in some shape what we will do with certain States, so as to induce them to cease waring."
As the same article notes, some Southerners were a bit more realistic: "Of all the humbugs of the age", one southern iconoclast wrote, "this Northwestern hobby is the most absurd, and at the same time the most dangerous. Newspapers, having exhausted themselves on the European intervention, are now trying to raise our hopes by the promise of a new alliance." Indeed, the extreme Peace Democrats who floated the idea of a Northwest Confederacy were probably not even a majority of the Democracy of the Northwest, let alone of the Northwest as a whole. Of course a much better military showing by the Confederacy could conceivably make the idea more popular in the Northwest, but to see the idea as a *substitute* for further Confederate victories on the battlefield (rather than as a possible if unlikely *result* of them) was unrealistic. However, the Indiana and Illinois legislatures had been won by the Democrats in 1862, and many of these Democrats were calling for the US government to hold a peace conference with the Confederates; in the minds of some Southerners, there was not much of a step from that to the Northwest holding a *separate* peace conference if Lincoln turned down the idea.