Just look at how much the majority opinion on dozens of topics has changed over the past few decades. Marijuana legalization. Gay marriage. Mass incarceration.
Lots and lots of change happens in the hearts and minds of lots and lots of people without force being involved. People often change their views very gradually, over time, as they realize aspects of things which weren't apparent to them before. A big part of this is often empathetic discussions with the people around them.
I think you, and other commenters in this thread, seem to imply that the assumptive goal is to change someone's mind RIGHT NOW, or else just force them to do what you know is right, and ignore their wrongheadedness.
The civil war was a serious net benefit to humanity by outlawing slavery. But the underlying concerns of Southerners, which was the dismantling of their economy, was never addressed. This has led to generational poverty in the south which has never been fully undone, and created a fertile field for the southern strategy and Jim Crow, which are arguably continuing to affect us in the form of the current presidential campaign.
Force may have been necessary, but a deeper understanding might have made the aftermath of force far better.
One of the first things to realise about people's apparently very strong views is that often they're simply echoing what they think is the majority view (but they generally really won't like you pointing that out). A second is that often they change quite swiftly when people stop talking about it. Not only are they not constantly being reminded that X is bad by people and sources they trust, but they also don't feel obliged to keep defending a position they held a couple of years ago when not regularly prompted to justify it.
The new majority opinion is held by a largely different set of people than the one from a few decades ago. People born in the 19th century still mostly think what they thought before, they're just too dead for it to matter.
Joeboy beat me to it, but I'll reiterate: that's not compromise. It's just change. Don't confuse the two.
And for those on the "losing" side of that change, there's an incredible amount of consternation.
I think that depends very much on the topic. For instance - there was once a point in my life where I was opposed to state-sanctioned same-sex marriage. My reasons for this were complex, and not based on a fear or hatred of homosexuals. Over time I came around on it, and by the time SCOTUS ruled on it I was supportive of it (though not of the legal reasoning they cited).
On the other hand, my views on gun rights have not changed, and I cannot envision a point where they would. If Heller were to be overturned I would not "accepting" of it in even the smallest way.
In other words some issues are core beliefs and some issues are secondary. Peoples views on secondary beliefs change over time, but core beliefs are not something that I have often seen changed.
However, when the topic gets brought up again a while later, they have shifted their position closer to mine, but also insist that they've always thought that.
I really wonder what is going on there, but I can see the aggregate opinion shifting by talking while nobody thinks they 'lost' the debate (for all those who identify far too strongly with their opinions.).
What is the appropriate life per slavery-year exchange rate? Because if for one less year of slavery you would spend 10,000 human lives, and 20 years of peaceful compromise would have ended slavery, then your war was a mistake.
There's no way to even estimate such things beforehand, so the calculation you propose is pretty useless.
I'm not saying that the decision for the war was right or wrong, but that you can't answer the question with a seemingly objective multiplication of numbers (it's the garbage-in-garbage-out principle in action).
So I'll make a counter-point: let's say we're back in the old slavery days. I'm sure Abe did his fair share of listening and talking with folks from the South.
But eventually, there just ain't no compromising and something had to be done.
And so it was done.
Ultimately, I think, for some issues, there is simply no room for compromise. At that point, it's simply a question of who has the power to push their agenda. That or there's gridlock and nothing happens at all (see: climate change).
Now, on a one-on-one basis, I think this post is full of excellent lessons. Active listening, empathic connection, these are great tools when dealing with individuals, whether that be in a personal or professional setting.
But in the large? To me, history demonstrates that it's simply idealism to believe that people can, in aggregate, come together on complex, polarizing issues.