Retired Marine Gen. James Mattis is a legend in the military. Revered by Marines and non-Marines alike, Mattis has taken on the persona of a modern-day Patton: having the knowledge and insight to lead his Marines through combat, while standing behind them and taking the heat if things go bad. In short, Mattis is a hell of a leader.

In 2013, Mattis retired as commander of Central Command in Tampa, Fla. after four decades of service. Since then, he’s been teaching at Stanford and Dartmouth, as well as speaking on leadership across the country. He’s also working on a book with author Bing West.

Now we look back at some of the best insights he has offered through a great collection of quotes. Most apply strictly to military service, but some can be just as useful in the corporate boardroom.

“You cannot allow any of your people to avoid the brutal facts. If they start living in a dream world, it’s going to be bad.”

The “dream world” Mattis is talking about is one of denial and complacency — a mood in combat that can get you killed. And in corporate America, it can get you wiped out by the competition.