The fact is the Civil War happened. 300,000 died trying to free millions. 300,000 died trying to keep them in chains. The result is they are free. That's a good thing.
Today, there are millions of Christians in the Sudan who are enslaved by their Muslim enemies.
1. Would it be worth "spending" 300,000 lives to free them?
2. Would it be worth "spending" 1 life to free them?
3. Would it be worth "spending" 1 life to free them if that one life were yours?
My guess is that your answers to these questions depend entirely upon how close these lives are to your own. It's easy to think in terms of abstractions by illustrating how many lives were lost in a "noble" effort to achieve a "positive" result and end slavery 140 years ago, but I am quite certain of one thing: If there were millions of blacks enslaved in Confederate states today, and the Union went to war to free those slaves, you and I wouldn't be found within a hundred miles of a Union Army recruiting office.
The American Civil War was communism's first major defeat. If the world had drawn the proper conclusions, we might have been spared a great deal of needless suffering in the twentieth century.