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.

“If in order to kill the enemy you have to kill an innocent, don’t take the shot. Don’t create more enemies than you take out by some immoral act.”

Mattis, who co-wrote the manual for Counterinsurgency with Gen. David Petraeus, knows well that troops cannot win the population over to their side if they are killing the wrong people. His advice here to soldiers and Marines is spot on.

“I don’t lose any sleep at night over the potential for failure. I cannot even spell the word.”

Of course he can spell the word but that’s not the point. Mattis wants to impress upon his troops that failure should not be an option.

“Be polite, be professional, but have a plan to kill everybody you meet.”

Before his Marines deployed to Iraq in 2003, he told them the above (along with many other great pieces of advice in a now-famous letter). His point here is to be a professional warfighter who can be polite with civilians, but always remember that if things go south, the dirty work needs to get done.

“The first time you blow someone away is not an insignificant event. That said, there are some sh–heads in the world that just need to be shot. There are hunters and there are victims. By your discipline, you will decide if you are a hunter or a victim.”

Recalling the mentality of the wolf, the sheep, and the sheepdog, Mattis understands that there is evil in the world. It’s important for his men to be prepared if they ever face it.