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.

“There are some people who think you have to hate them in order to shoot them. I don’t think you do. It’s just business.”

One of his more controversial quotes, to be sure. But in Mattis’s view, to be a professional, you need to have a professional mindset. It’s not really necessary to get emotional about what you have to do. It just needs to get done.

“You can overcome wrong technology. Your people have the initiative, they see the problem, no big deal … you can’t overcome bad culture. You’ve gotta change whoever is in charge.”

In a talk at Stanford, Mattis was relating how toxic culture can bring down an organization that has everything else right. The culture of an organization comes from the top, and if that part is screwed up, there are going to be problems.

“The most important six inches on the battlefield is between your ears.”

Mattis doesn’t want robots just mindlessly following his orders. As a leader, he gives broad guidance and lets his men use their own brains to decide how it gets accomplished.

Amen.

“In this age, I don’t care how tactically or operationally brilliant you are, if you cannot create harmony — even vicious harmony — on the battlefield based on trust across service lines, across coalition and national lines, and across civilian/military lines, you need to go home, because your leadership is obsolete. We have got to have officers who can create harmony across all those lines.”

Mattis implores his officers to not get stuck in their own little boxes. Learning how to be brilliant on the battlefield is important, but it’s more important to be able to work with others to get the job done.

“PowerPoint makes us stupid.”

Military officers endure (and have to create) tons of PowerPoint briefings to inform their chain of command of what’s going on. Mattis however, is not one of those officers. He actually banned PowerPoint since he saw it as a waste of time.

“You are part of the world’s most feared and trusted force. Engage your brain before you engage your weapon.”

Mattis wants his Marines to always be thinking before they take the shot. It’s advice that has no doubt saved lives.

“An untrained or uneducated Marine… deployed to the combat zone is a bigger threat to mission accomplishment… than the enemy.”

The biggest detriment to mission accomplishment is not from the competition, but from within. Having the right mindset and skills is what results in getting results.

“No war is over until the enemy says it’s over. We may think it over, we may declare it over, but in fact, the enemy gets a vote.”

Combat doesn’t happen in a vacuum. All the planning, meetings, and briefings on what potentially can happen in a given situation are necessary, but the bad guys will always react in uncertain ways. The key is to be prepared for anything.

“Be the hunter, not the hunted: Never allow your unit to be caught with its guard down.”

Just because you are at the top of your game doesn’t mean someone won’t come along to knock you down. Units (and individuals) need to be vigilant and make sure that doesn’t happen.

“Ultimately, a real understanding of history means that we face NOTHING new under the sun.”

Mattis is an avid reader. On all his deployments, the general brought along a ton of books that he thought might help him along the way. In an email that went viral (via Business Insider) on the importance of reading, Mattis wrote that it “doesn’t give me all the answers, but it lights what is often a dark path ahead.”

“You’ve been told that you’re broken. That you’re damaged goods … there is also Post-Traumatic Growth. You come back from war stronger and more sure of who you are.”

While giving a speech to veterans in San Francisco, Mattis tried to dispel the mindset that those leaving the service should be pitied. Instead, he told them, use your experiences as something positive that teaches you to be a better person.