Military Intelligence

What is military Intelligence? Intelligence is the subject of many myths and misunderstandings. It is not the same thing as "spying", but it may use information from spying. It is not even just about "secrets", but most of what it does will be secret. So if that is what military Intelligence is not, then what is it? Personally I feel that the Duke of Wellington (famous for defeating Napoleon at Waterloo) said it best:
'All of the business of war, and indeed all of the business of life, is to endeavor to find out what you don't know by what you do.…