When did Roman Empire begin to decline and stop expanding?

It's important to realise that the Rome Empire was a period encompassed by Ancient Rome. Ancient Rome refers to the very beginning of the city of Rome, founded in the 8th century BC, and it's expansion, through to the collapse of the Western Roman Empire in the 5th century AD.

Did the Roman Empire expand?

Rome was able to gain its empire in large part by extending some form of citizenship to many of the people it conquered. Military expansion drove economic development, bringing enslaved people and loot back to Rome, which in turn transformed the city of Rome and Roman culture.