After the Pax Romana, the Roman Empire started to decline. The reasons were political and economic, and they had problems with foreigners. After the guard killed Commudus, they sold power to the highest bidder. This led to chaos every time a leader died. All leaders became dictators, and almost all were killed by their guards. The Roman Empire had 26 leaders in the next 50 years.

A leader had to pay his army well to keep them happy so that they would fend off invasions. This led to high taxes and inflation. Borders were left open as Romans fought among themselves. Germanic tribes invaded, Persians took Roman lands, and Gaul tried to become independent.

Thats why

So they got to big for their togas. i think. I only read in to the emperor that started it all. Octaviun, aka Agustas Ceaser. It all started with Night at the Museum 1

Actually, if you're referring to the book published, it's "The Decline and Fall of the Roman Empire"... It did fall, not just decline, after all...