The Civil War was a major defining conflict for the United States, one of the simplest ways to describe the change in the United States was how the average citizen referred to the nation, prior to the war it was often called “these United States” and after the war it changed to “the United States.” But forging that new sense of unity involved a considerable amount of blood and stretching the powers granted the federal government under the Constitution to hold the various parts of the nation. In particular President Lincoln throughout the war made a point of exercising “expansive” federal powers in the Midwestern states due to a strong pro-South, pro-Democratic party leaning in the region. Lincoln, although not directly approving extreme actions, often allowed by inaction military commanders to take extreme steps to keep the region loyal, including using intimidation tactics, targeted arrests, suppressing the press, and expelling dangerous political figures to ensure that the American Midwest remained solid in its allegiance. This in turn sparked its own problems, mainly the growth of groups that advocated separation from the United States and the formation of a new third nation from the states of Ohio, Indiana, Illinois, and Missouri.
The center point of this plan was a combination of a local organization that called itself the Sons of Liberty (hailing back to the American Revolution) and lead by Clement Vallandigham (pictured above) working with Confederate raiders to enact a complicated plan in 1864 to split these four states from the rest of the union and create a new nation, the planned working name for this new entity would be the Northwest Confederation. (The name hails from the regions original designation in the early post-Revolution period as the Northwest Territory.) The plan was ambitious in its goals – Confederate cavalry raiders would head into the state of Illinois to link up with Sons of Liberty militia units – the combined force would liberate a series of Confederate prisoner of war camps and arm the freed soldiers with weapons taken from state arsenals. This newly combined force, it was hoped, would total over 100,000 soldiers in arms and provide enough force to spark other pro-South leaning individuals to join the effort and create a new nation. This was all to start at the Democratic Party National Convention in Chicago.
The plan collapsed though, a combination of secret police/spies loyal to the federal government discovered the plot and arrested a few key leaders, but mainly internal bickering and the fact that most Sons of Liberty when faced with the call to actually rise up in arms against the federal government and the other Union states backed out of the plan. It did have one lingering impact though, from 1864 till the Spanish American War the Republican Party was able to bring up this event to brand the Democratic Party as the party of “traitors and backstabbers.” It was one more effective election tactic that helped ensure the Republicans maintained a dominant political position in the United States for nearly twenty years.
Sources: Encyclopedia.com entry on the Northwest Conspiracy, Wikipedia entry on Clement Vallandigham, The Northwest Conspiracy by Thomas Fleming in What Ifs? of American History edited by Robert Cowley.