Answer:
During the Pax Romana, wars were rare and Rome remained a united, relatively peaceful area- in fact, the term "Pax Romana" literally means "Roman Peace." Rome was not defeated in any battles, and most fights were over land increase, and not much else.
After the Pax Romana, Rome began to dissolve and become less powerful. The emperor and other people in political power became less interested in ruling well, and more interested in just plain power. The position of emperor was sold to the highest bidder by the royal guard, and there was little money left to defend the Roman borders from barbarians, and keep the roads clear and safe, because all the money was being used to pay off the army and mercenaries that had to be hired, was misused, or went to building Christian churches and buildings.