When Rome defeated tribes throughout the time of the empire, they never seemed to eliminate them in genocides whenever Rome won. Instead, Rome plundered them and sent them home, or even provided them with land in roman territory. How come? Was it because Rome was afraid that a large portion of the tribe, keeping themselves safe outside of Rome during an invasion, would retaliate? Or was it because they were afraid to inspire other tribes, and even inspire them to make genocides against the romans? Or was it simply because of moral / traditional reasons?
Of course it would have been horrible, especially in today's eyes, but why wasn't it a rational solution back then?
Hi OP!
Great question! Though before I can dive into it, I have to bring up two points. First, almost all surviving sources we have are Roman, and they are of course invested in portraying Rome in the best possible light (even self-critical ones are still examples of an imperial centre examining itself, rather than an outside view). Second, we need to define genocide – it would be all too easy to over-intellectualise the question by falling behind changing definitions of what constituted genocide (or what even is race/genos). To keep it simple, let us say it is the concentrated destruction of a people and their way of life.
Going on that, there were numerous examples of this happening throughout Rome’s reign, republican or imperial. As early as Camillus’ conquest of Veii, destruction and slavery was the norm. Camillus, taking Veii (396 BCE), killed the entire male population and enslaved the women and children.
During the siege of Carthage in the Third Punic War (146 BCE), Scipio Africanus massacred the city for six days and took only 50,000 prisoners (all of whom he sold into slavery). Carthage itself was burnt to the ground and was not rebuilt until the time of Julius Caesar. In the same year, Lucius Mummius sacked the Greek city of Corinth. The city was too set on fire, all male inhabitants slaughtered, and the women and children sold off into slavery. Polybius, an eyewitness to the destruction, would also record how the soldiers looted every piece of art. Julius Caesar’s famous Gallic War, by the ancient sources (Appian, Plutarch, etc.), led to 1 million Gallic deaths and another million enslaved. Even though this was likely exaggerated, the numbers are still mind-blowing.
This did not change much under the emperors. The Pannonian Revolt under Augustus (6 – 9 CE) led to the extermination of entire peoples (see Valleius Paterculus). Suppressing the Jewish Revolt, the future emperor Titus destroyed Jerusalem (70 CE). Josephus states that upwards of 1.1 million people were killed (due to many Jewish people coming to Jerusalem for Passover), with only 97,000 taken prisoner and enslaved (modern scholarly estimates are much lower, but still more than 300,000 killed). All Jewish people were expelled from Judea and in place of Jerusalem a new city, Aelia Capitolina, was founded, complete with a temple to Jupiter standing on the Temple Mount. Numerous scholars have interpreted this as a concerted effort to destroy the Jewish connection to the land. Trajan, during his invasion of Dacia (101 – 106 CE), levelled the entire capital city of Sarmizegetusa. Decebalus, the King of the Dacians, feared capture by Rome so much that he chose suicide.
For Rome, the standard process in war was conquest and enslavement. It must be noted that, by the end of the republic and the early empire, slaves, most of whom would have come from captives in war, made up upwards of 40% of the population of Italy. The Roman empire would have required hundreds of thousands of new slaves each year as replacement.
If lucky, an opposing nation could hope to sue for peace without total destruction. But during the sack of a city, Rome was anything but kind. I would argue that times where extreme leniency was shown, while not impossible, were more likely than not to have been propaganda. Rome committed many horrible acts in war, though routine for those times, which would have easily been considered genocide had they been committed today.
The number of scholarly sources written on the military history of Rome is far too extensive to list here. For some good summaries, check out the Edinburgh History of Ancient Rome series. Reading the ancient sources directly is also helpful, particularly those of Livy, Cassius Dio, and Polybius.
I'm not 100% sure I answered your question fully, but I hope it helped!