Why did the Roman Empire grow so large and then fall apart?

Once upon a time, a single city on a river called the Tiber grew into the biggest empire the ancient Mediterranean world had ever seen. It stretched from rainy Britain all the way to the warm sands of Egypt. So how did one city swallow so much map — and then, centuries later, slowly let it slip away? Let's follow the story.
Rome's first secret weapon wasn't a weapon at all — it was roads. Wherever Rome went, it laid down long, straight stone roads, like veins carrying soldiers, traders, and messages across the land. An order could travel from the capital to a distant fort in days instead of months. When you can move fast, you can hold a lot of ground.
