Is Morale the Most Important Thing on a Strategic Battlefield?

Morale is one of those things that on a tactical level, on a battlefield level, is undeniably huge. The idea is not to kill everybody on a battlefield. Traditionally, it's to kill enough people to break their will to continue fight. To destroy their morale, killing as a means to an end. In the second world war, some people say you can. Some people says you can't. But they've been writing about it military strategists since the bronze age. What breaks a nation's morale? And what proof do you have that that's something that even occurs?