Retired General Stanley McChrystal, a former head of the Joint Special Operations Command (JSOC) and commander of all forces in Afghanistan, recently co-wrote a book on leadership, exploring what makes an effective leader.

The book, “Leaders: Myth and Reality,” explores a wide range of leaders across centuries of human history. The authors’ selection of leaders isn’t limited to military men. They explore the leadership traits and styles of business owners (such as Walt Disney and Coco Chanel), artists (such as Leonard Bernstein), politicians (such as Margaret Thatcher), scientists (such as Albert Einstein), civil-rights advocates (such as Martin Luther King Jr. and Harriet Tubman), and even terrorists (such as his former opponent Abu Musab al-Zarqawi, the bloodthirsty founder and leader of al-Qaida in Iraq). One of the more prominent leaders the book examines is Confederate General Robert E. Lee, a personal hero figure for McChrystal.

The book’s overall thesis is that different environments necessitate different styles of leadership and that people tend to choose leaders they need. Furthermore, McChrystal and his fellow authors assert that current and prospective leaders shouldn’t endeavor to memorize a list of leadership qualities but rather to understand how to interpret different situations and then determine what sort of leadership is required.

McChrystal’s co-authors are Jeff Eggers, a former Navy SEAL and special assistant for national security affairs to President Obama, and Jay Mangone, a Marine Corps veteran and experienced researcher currently serving as principal researcher at the McChrystal Group.

Speaking about the book at the headquarters of the Association of the U.S. Army, McChrystal said about leader advancement that “our mindset is if you weren’t at the wedding you really shouldn’t be here. I think that’s a mistake nowadays. I think we should be taking people in laterally. And that’s very threatening to people that are in the guild.”

He even recommended a rather unconventional proposal with respect to today’s military and leader development and evolution. Asked about the plan to have civilians with high-demand skillsets (cyber warfare, for instance) enter the military at high ranks, he said “the military should be taking people through lateral entry at every rank; they should be bringing in not just specialists in cyber to do a thing. We’ve seen people come in pretty senior and be extraordinary warriors back in history.”

McChrystal also stressed that it’s the situation that enables a leader to rise and be effective. Of course, he or she has to have some—but not all—of the right values, such as insight, prudence, communication skills, vision, and physical and moral courage.

The former JSOC commander also emphasized the need for good critical thinking. Addressing the type of leadership America needs, he said that, as responsible citizens, we ought to make that decision based on our individual values and a concern for the common good rather than having someone else telling us what we need.