Before the Civil War they said freeing the slaves would ruin the economy. The US had to free them because it was a moral imperative. The war cost us 5% of the US population in casualties. The sum total of the monetary value of all slaves at the start of the civil war was roughly one trillion dollars in today's dollars. The slaves were freed and the US became the world's greatest economic power as a result.
Really? The US didn't become the number one economy until 1916 - about the time that most of the European powers (specifically, the UK - which was the biggest economy before the US stepped into that role) were deep into World War I. World War II pretty much cemented our position as the rest of the first world (and much of the 2nd) was bombed and broken. I don't think it was the slaves that made us the greatest economic power, but rather the fact we have two large bodies of water keeping us relatively safe from wars in Europe and Asia.