John Boyd was a person who was really good at thinking about how soldiers can win battles. He was like a coach or a teacher who taught a lot of people in the army how to use their brains to do better in fights. He was a big deal in the 20th century because he revolutionized the way military strategists thought about combat.
John Boyd believed that the key to winning is not always having the best weapons or the biggest army. Instead, he believed that it is more important to have a good plan and to be able to change that plan quickly when things don't work out. This is called agility, or being able to move quickly and easily.
Boyd also made a very famous diagram called the OODA Loop, which stands for Observe, Orient, Decide, and Act. It is a way of thinking about how people make decisions in stressful situations, like battles. Boyd said that if you can cycle through this process faster than your opponent, you'll usually win the fight.
Overall, John Boyd was a very smart man who taught the military how to think about fighting in a smarter way. He believed in being agile and quick-thinking and created a lot of ideas that are still used today by military strategists all over the world.