This excellent blog series [0] goes in to a little more details on the Roman Conquest, in the specific context of many cultures glorifying the primitive other as being morally pure and more capable fighters. Genocide is the only word to use. I previously had no idea just how, erm, effective the Romans were.

> historical events that could've completely changed the course of history

The battle of Teutoburg Forest [1][2] is another one. Had the Roman general Varius not fallen in to Arminius ambush, things might have been vastly different. The Romans never again had a solid footing beyond the Rhine and the battle was used again and again to forge German identity.

> One thing he notes is that the closer someone is to someone, the harder it is for them to (psychologically) kill them.

This same effect is discussed extensively in the book On Killing [3] and similar conclusions can be drawn from the Milgram Experiment [4]. On Killing also mentions that stabbing weapons are psychologically harder to use than slashing weapons. It specifically mentions the Roman Gladius.

0 - https://acoup.blog/2020/01/17/collections-the-fremen-mirage-...

1 - https://www.bbc.co.uk/programmes/m000f69q

2 - https://en.wikipedia.org/wiki/Battle_of_the_Teutoburg_Forest

3 - https://en.wikipedia.org/wiki/On_Killing

4 - https://en.wikipedia.org/wiki/Milgram_experiment