Answer:
The were a number of underlying false assumptions that led the North astray. The biggest one was that the North assumed the vast majority of slaves would support the North's invasion of the South. However, the South had the idea of manumission for its slaves as opposed to the North's idea of emancipation of its slaves.
Explanation:
There were actually two different brands of slavery still in existence. The decidedly British, old world brand with its well known abuses, and the New World French and Indian hybrid version of enslavement and voluntary servitude where such British abuses were prohibited.
In the French homeland, slavery was abolished long before 1776. Southerners had already naturally adapted to the French New World brand of slavey prior to the Revolutionay War, that's why France had no problem supporting the birth of the United States.
However both the North and South missed the idea that they were saying two different things with the same words, even the same numbers. Similar to Winston Churchill's idea that the US and the Brits are two nations separated by a common language.
There are many historical references of Northern soldiers writing home, totally freaked out that the South had Blacks supporting the Confederacy. The idea was totally foreign to the North. With an insufficient percentage of southern slaves supporting the North and its version of slavery the brief Civil War the North envisioned was doomed before it started.
