Government under the roman republic or government under the roman empire to be more successful an why

He Roman Republic was what made Rome great. The Roman Empire was what brought about its decline and fall.

When Rome became a Republic in 509BC, Rome's territory consisted of little more than the city itself. Rome was barely more than a collection of villages. By the time the Republic was effectively abolished by Octavian at the end of the 1st century BC, Rome had become a huge empire, as this map shows.