Well, that depends on your own findings. My research on this issue, from several historians, is the following version (Although it is quite summarized and condensed, of course):
The molossian dog (the base for all mastiff type of dogs) is believed to come from Asia. These molossian dogs were greatly appreciated by Greeks and Romans... some historians say that the Tibetan mastiff was brought to Greece by Alexander the Great. But is believed today that the Molossian dog was brought to England by Phoenician sailors and that this animal became the ancestor of the English mastiff. The Neopolitan mastiff is the Italian version of the Molossian of the East (Former Persia, now the Iran-Iraq region). Attrition from a combination of poor breeding practices and life during the middle ages nearly demolished this breed. Sicilian Kings brought it back by mixing the Dogue de Bordeaux (French mastiff) with local Italian ancient mastiffs. The Dogue of Bordeaux is also a truly descendant of the molossian Asian dog and a has a proven history too.
The Rottweiler, as well as other mastiff type of dogs, is descended from the molossian of the Tibet (Tibetan mastiff). The Romans had herding dogs and household guardians, and they highly praised this molossian dog from the Tibet that it is believed was crossed with other ancient dogs, Thus, the Rottweiler base line was born. There is documented evidence of two Rottweilers carved in stone upon a portal of a Roman home (Now Italy). These proud robust dogs were taken to Great Britain during the Roma Empire conquest, and remained there... but, in time, they finally found their way to Germany, concentrated particularly in the town of Rottweil. Legend holds that men and dogs drove their herds to the market as equals in the task. However, highwaymen were a real threat in those days and the butchers placed their full purses upon the necks of the dogs. thus did the Rottweiler's reputation as the best of all guardians came into being. No thief would risk life or a limb to challenge them Rottweilers! Towards the close of the 1800's cattle drives by dogs were replaced by the railroad, and so the Rottweiler found himself "out of work". It is said that in 1900 there were only a couple of Rottweilers could be found in the town of Rottweil, Germany. Within a few years the qualities that made the Rottweiler the king of guardians allowed him a new occupation: police work. In the early 1900's the Rottweiler was admitted by the German Police Association as one of the very few dog breeds accepted for the task... and the rest is "history"!... So I think we can agree that Germans saved the Rottweiler from dissapearing altogether
And what about the so-called "oldest coursing mastiff on Earth", the Cane Corso?... It is a fabricated story from cross-breeding the Neopolitan mastiffs with Rottweilers in the early 1980's... and then brought in from Italy to the USA by Tom Sottile, in the late 1980's, as this "rare-forgotten" new (?!) breed that nobody knew about it before!!!... Yeah, right.