Oswald Spengler was a grown-up who lived a long time ago and had some very interesting ideas about how countries and civilizations work. He thought that just like people are born, grow up, get old, and die, countries and civilizations also have a similar life cycle.
Spengler believed that countries have a sort of youth period where they are developing and learning new things, then they hit their prime where they are strong and successful, and eventually they will start to decline and fall apart.
He also thought that different civilizations from different parts of the world go through these life cycles at different times. For example, he believed that ancient Roman and Greek civilizations were both very successful, but they eventually declined and disappeared while the civilization of Islam from the Middle East was just reaching its prime at the same time.
Spengler's ideas were very interesting, and many people still study them and talk about them today. He made us think about how countries and civilizations change over time and why they do.