Here's what legendary Marine General James 'Mad Dog' Mattis is really like, from people who served with him

President-elect Donald Trump just tapped Retired Marine Gen. James Mattis for Defense Secretary, so it's worth asking what kind of leadership the Pentagon is in for.

As it turns out, according to a number of those who served with him, the more than 3 million civilian and military personnel of DoD can expect a well-read history buff with a strategic mind, a senior man who is not above talking to even the most junior personnel, and a sometimes gruff, opinionated leader who isn't afraid to tell it like it is.

Business Insider spoke with a number of people who served with Mattis, and gathered up other anecdotes, to understand what the former four-star general is really like when he's in charge.

Here's what we learned.