What does you can lead a horse to water but you can't make it drink mean

People, like horses, will only do what they have a mind to do. or. Something that you say which means you can give someone the opportunity to do something, but you cannot force them to do it if they do not want to. I hope this helps

I believe it means that you can give someone the opportunity to do something, but you can't force them to do it, if they wish not to. It can also mean that people, like horses and other stubborn animals, will only do what they have a mind to do. 

What’s the choices

Because the vietnam war was fought in an especially brutal and untraditional way