> On the Roman side there were zero casualties so it was more a genocide than a battle.

While it's certainly true that it meets the definition of a genocide, it's rather anachronistic to call it that. Ancient peoples did not possess our norms and values.

To the Romans, the Germanic and Celtic peoples represented an existential threat. And they were correct in that analysis. Periodically, for hundreds of years, German tribes the Romans had never even heard of came down into Italy in mass migrations looting and killing everyone in their path. The numbers involved were always absolutely massive, and in many respects the Germans were superior fighters. On several occasions the Romans survived only by the skin of their teeth. For various reasons, the cultural and demographics of the Germanic tribes provided them with a huge birthrate and rapid population growth, which led to pressure that eventually led to huge armed migrations. (Of course, war for pleasure and profit were also common, and later often they fled from even more numerous, warlike tribes, Germanic or otherwise.)

To say, as an aside, that 'among these were women and children' is also a little disingenuous and anachronistic. Not only were women and children not regarded as particularly sacrosanct or immune from the horrors of war, the Germanic and Celtic peoples often included their women in their armies on purpose to prevent the men from retreating, to assist in whatever ways they could, and sometimes to fight. Sometimes the women in the rear merely mocked or pleaded with the men not to retreat. Sometimes they killed them.

I confess I am not intimately familiar with these particular tribes and this particular battle. But from a realpolitik point of view, Caesar was acting sensibly. Letting these tribes survive would be to the eventual detriment of Rome. They would eventually invade. No agreement with the Germanic peoples ever lasted long. Nothing less than genocide ever kept a tribe suppressed for more than decades or centuries.

In general, ancient peoples had a very different mindset than we do. Most of them regarded their ethnicity and culture as an absolutely bedrock part of their identity. For many of them, it was better to die as and for who they were than to live in submission - even peaceful submission - to a foreign people. This was true for both the Romans and many of the Germanic tribes. The Cimbri women, for example, killed their children and themselves rather than submit to the Romans. Indeed, the particularly strong Roman identity is one of the reasons for their early successes. The Romans often suffered devastating losses, both in absolute numbers and relative proportions of the Roman people, and refused to surrender or stop fighting where many other peoples did. We could probably, anachronistically, call the Romans and the Germans genocidal racists. But the only reason the Romans survived to become an empire and provide future generations with all their advancements and civilizations was because they were genocidal racists.

Of course, as time went on, and Rome became an empire, 'Roman' became more of a national identity and less of an ethnic or cultural identity. This was correlated with what Roman conservatives thought of as mass decadence: for example, the importation of foreign values, dress, and behaviors. The conservative outrage against homosexuals and transgendered people almost reads like something from the modern day. And the truly 'Roman' part of the population became less of a factor due to lower birth rates (Romans had birth control and abortion, and probably also due to the increasingly large role of women in society). Perhaps consequently, Romans began seeing the military as less of a duty and more of a burden, particularly in conjunction with ever-ongoing wars, and the Roman military essentially became a mercenary army.

As time went on, German pressure on Roman borders became intolerable. Rome constantly meddled with the tribes to play them off against each other, but Germanic peoples remained a perpetual problem for the empire. Since Germans were incredibly effective fighters - born to military life, and often bigger and stronger than Romans, particularly as the empire became more 'decadent' and commercial and academic activity became more common than warfare for the average citizen, Romans began to incorporate Germans into the military, both as auxiliaries and as actual members.

Then as Roman internal politics became increasingly complicated due to the changes in the governance structure, the economy, and interminable civil wars, the Roman military became less effective and the Romans often turned a mostly blind (and helpless) eye to Germanic migrations/invasions and sometimes encouraged them. Then, after several more crises, the Romans allowed a contingent of Germanic refugees fleeing from the Huns into the empire. Ultimately these Germans (the Goths) turned against the Romans (perhaps justifiably) and that was one of the primary proximate causes of the end of the western Roman empire.

...I suppose I went on a little there, but I like the subject and I recently finished Decline and Fall of the Roman Empire. :)