I’ve recently stumbled upon the claim that Cato the Younger, upon hearing of Caesar’s war with the Germanic Usipetes and Tencteri peoples, called upon the senate to send Caesar in chains to the Germans. However, I have only twice in my googlings found any source for the claim: the original one from Livius.org, and the second from a TIL reddit post- referencing this same page. So is there any PRIMARY source for this event?
Of course. BTW, Livius.org is a fine site as far as it goes. I wouldn't put it in a paper, but it can 100% point you in the right direction. For instance on the page you linked, it quoted Caesar at length and identified the translation used.
The primary source for this is Plutarch, Cato the Younger. The text is from a similarly useful site, [LacusCurtius] (http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Cato_Minor*.html) at section 51:
After Caesar had fallen upon warlike nations and at great hazards conquered them, and when it was believed that he had attacked the Germans even during a truce and slain three hundred thousand of them, there was a general demand at Rome that the people should offer sacrifices of good tidings, but Cato urged them to surrender Caesar to those whom he had wronged, and not to turn upon themselves, or allow to fall upon their city, the pollution of his crime.
It's also in Plutarch's Caesar, section 22:
But Tanusius says that when the senate voted sacrifices of rejoicing over the victory, Cato pronounced the opinion that they ought to deliver up Caesar to the Barbarians, thus purging away the violation of the truce in behalf of the city, and turning the curse therefor on the guilty man.
We get Plutarch's source here, this guy Tanusius. Unfortunately we know almost nothing about this guy aside from this comment. The OCD only says
anti-Caesarian historian (and senator?) of the late republic, used by Plutarch [in the Caesar passage I cited] and by L. Suetonius [Life of Julius]. L. Anneus Seneca speaks of his Annals as 'heavy going'.... Probably of Etruscan origin.
You'll note, though, the charge Cato levied wasn't genocide. That term was coined in 1944 and there are people around here far more familiar with the development and use of that term than I; the point is Cato couldn't have made that accusation. And Plutarch doesn't say he did. Plutarch says Cato was upset by Caesar's violation of a truce.
In fact, I think a careful reading of Livius.org's text would show how genocide works in that context. Here's the text again:
The incident illustrates the ruthless nature of Roman aggression: Caesar first provoked the refugees and finally attacked them during an armistice. When the genocide became known in Rome, Senator Cato the Younger exclaimed that Caesar, the general of eight legions, was to be handed over to those Germans.
The first sentence states the fact the way Plutarch states it. Caesar attacked under a truce. The second sentence invokes the word "genocide," but there's nothing in the introduction to the long Caesar passage quoted to connect an attack under truce to systematic extermination of a populace. It has to be an editorial comment of the author.