The Roman armies used mastiff-type dogs to do battle, to frighten intruders, and to herd and guard their walking storehouse of cattle (their food) while they invaded "Europe." The dogs that were the closest thing to the modern-day Rottweiler were the "cowhands," herding and guarding the cattle for the encroaching Roman armies.
These were fierce animals. They had to be. Not only did these early Mastiff/Rottweiler types have to control the cattle, they had to fend off wolves and other dangers, the largest danger being those who resisted the Roman invasion.
But they had to be loyal to their own. They had to distinguish between an enemy and an ally. And the Mastiff/Rottweiler has kept that gift of discernation down to the present day. And that "gift" is evident in the modern-day Rottweiler: A Rottweiler will not attack his own unless severely provoked; he will not attack a visitor if he's been properly socialized that "visitors" are not the enemy; a Rottweiler will protect everything and anything on the property and this includes birds and squirrels.
To understand the Rottweiler, one must understand where they came from. They are not "attack" dogs, they are "protection" dogs. And they are loyal to the point of giving their own life to protect -- even if it is to protect a bird, or a squirrel on "their" property.
After the Roman armies left "Europe" -- and they left their dogs behind, too -- people began to notice the qualities in those dogs. And harnessed those qualities for their own.
The dog that eventually became the "Rottweiler" -- rott weil means "red tile" -- was embraced by the farmers in that region of Germany (Rottweil, where they produced red tiles). They used and developed every positive quality about him.
The Rottweiler became "the farmer's dog." The farmer lauded him for his herding abilities. The farmer used the Rottweiler to cart his produce (meat) to market; and the farmer tied his money on the Rottweiler's neck to get past the robbers on the way home. The Rottweiler was in his heyday.
Unfortunately, some time later, the German government made a decree that dogs could no longer be used to "go to market" and the demise of the Rottweiler began. Fortunately, there were those who had such affection for the breed that they refused to let the Rottweiler decline into extinction.