The main thing to know about him is that he’s a maverick who has repeatedly risked his Army career to do what he thinks is right. He made a name for himself in 1991 as a 28-year-old tank troop commander in the now-famous Battle of 73 Easting during Operation Desert Storm. His orders authorized him to advance only to the 70 Easting gridline on the map, but when a lieutenant told him they’d reached the limit of advance specified by their superiors, with the enemy on the run, McMaster replied, “Tell them we can’t stop. Tell them we’re in contact and we have to continue the attack. Tell them I’m sorry.” Result: McMaster’s unit, with only nine tanks, destroyed 57 Iraqi tanks, 28 infantry fighting vehicles, 11 light-armored tracked vehicles, 45 trucks, and three air defense artillery pieces. The U.S. suffered no casualties. The whole thing took 23 minutes.

Later, as a major, while getting his PhD at the University of North Carolina, he wrote a book called Dereliction of Duty: Lyndon Johnson, Robert McNamara, the Joint Chiefs of Staff, and the Lies that Led to Vietnam. That doesn’t sound like a book that would win friends in the military, but it’s now regarded in military circles as a classic.

Time named McMaster one of the 100 Most Influential People in 2014. In his profile, retired Lt. Gen. Dave Barno wrote that “the outspoken McMaster was passed over twice for selection for his first star. I watched senior Army generals argue over ways to end his career.” His fans won the day, obviously, and now, with three stars, McMaster is the first active-duty military officer to serve as National Security Adviser since Colin Powell and John Poindexter in the Reagan administration. This independent thinker’s performance in the Trump administration will be fascinating and instructive to watch.

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