What if the civil war did not take place?

In a final burst of summer reading -- for fun -- I just finished Ben H. Winters' Underground Airlines, a thriller in which a former slave, now serving as a U.S. Marshal, is sent to capture an escaped slave. The intriguing premise of the novel, however, is that the time is the present, and the civil war never took place. Instead, Congress in early 1861 passed a series of Constitutional amendments allowing slavery in a limited area of the country and forbidding any subsequent amendments abolishing slavery. Winters describes a country where slavery was confined to a "hard four" southern states, with trade and travel strictly controlled, and with much of the rest of the world boycotting goods linked to the slave states.

That counterfactual history sent me into my basement, searching for civil war books. Therein I learned that a proslavery compromise almost happened. Of course it didn't, and historians depict the last-minute efforts as futile. But I know that votes are votes, and even unintended legislation can have huge consequences.

So consider, on January 16, 1861, five proposed  amendments to the Constitution were defeated in the Senate by a narrow 25-23 vote. The measures, authored by Kentucky Senator John Crittenden,would have prohibited slavery only north of latitude 36, 30 minutes and allowed it in perpetuity elsewhere.

Even more surprising was a proposed 13th amendment to the Constitution that actually passed both House and Senate by 2/3 margins and was sent to the states for ratification. This measure, called the Corwin Amendment after its Ohio sponsor, would have prohibited any future amendment limiting slavery where is currently was legal. The actual text:

No amendment shall be made to the Constitution which will authorize or give to Congress the power to abolish or interfere, within any State, with the domestic institutions thereof, including that of persons held to labor or service by the laws of said State.

Only 3 states ever ratified it, and by then the civil war was raging. But what if...