Why did the Roman empire collapse?

The fall of Rome is usually considered to have taken place
in 476 A.D. when Odoacer, a minor German chief, captured Rome and forced the emperor off
his throne.  But Rome began to decline centuries before
that.

A time of peace and prosperity, known as
the Pax Romana, began in 27 B.C. when Augustus became emperor and it lasted until 180
A.D. with the end of the reign of Marcus Aurelius.  During the Pax Romana, the
government of Rome was stable, and the Roman Empire grew in wealth and power.  This came
to an end in 180 A.D. when Commodus, the son of Marcus Aurelius, became emperor. The
decline and the ultimate fall of the Roman Empire began with the end of the Pax Romana.
The decline of the Rome continued when the Empire was permanently divided into the
Eastern (Byzantine) Empire and the Western Roman
Empire.

There were several reasons for the decline and fall
of the Roman Empire.  First, there were political reasons. There were no rules on who
was to inherit the throne. This led to periods of civil war that plagued the Empire. 
Between 234 A.D. and 284 A.D. Rome had 26 different emperors. The division of the empire
also hurt the western empire. The best generals and administrators were found in the
east. Next, there were economic reasons.  More and more money was needed by the emperors
in order to pay the army and stay in power. This meant higher taxes which made Romans
unhappy and less loyal to the Empire. In addition, no new lands were conquered depriving
Rome of new revenues.  Eventually, inflation became a big problem.  The finally, there
were foreign enemies.  While Romans bickered and fought over politics and money, they
left the frontiers open to invasion. It was the invasion and capture of Rome by Odoacer
in 476 that is considered the fall of Rome.

 The beginnings
of the decline, or “falling”, of the Roman Empire could probably be traced back to the
reign of Commodus and the end of the Pax Romana in 180 A.D.