During the Roman Empire, Latin contributed to the cohesion among all the different countries which were under the Roman power.
Latin had a strong unity and was a vehicle of communication among people from different places.
In the 5th century, the decline of the Roman Empire started. So, the relations among such different countries broke down. Then, diversity tendencies inside the Empire increased and popular Latin (spoken Latin) lost the unity that had had before. This would be the origin of the future Romance languages. This process was slow (it took some centuries) and gradual.
Diagram of evolution:
Since the Romance languages have been consolidated, they have influenced themselves all over the years.
Romance languages can be divided into two big groups, because of their specific linguistic tendencies.
Copyright Marta Ribas 1994