President Donald Trump just forced out Defense Secretary James Mattis two months early, after Mattis criticized Trump for betraying allies.

Over the past two years, the relationship between Trump and Mattis has deteriorated. Mattis was once one of Trump's most highly decorated cabinet members. But Mattis ultimately resigned after Trump decided to pull American troops out of Syria.

Upon Mattis's nomination in 2016, the Defense Secretary was highly regarded as a masterful leader and scholar of war. According to a number of those who served with him, he's 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.

Jacob Shamsian contributed to this report.