I was actually just thinking about this earlier today, but I would go back further. Kill Leonidas, the Greeks never unify against the Persians, who claim all if Europe and allow local cultures/religions to self-govern. Celtic civilization thrives in western Europe, Gothic civilization in Northern Europe, all the other early European cultures never get replaced by Roman and later Catholic hegemony.

In fact, Catholicism never exists either because Jesus was born in a Persian Levant where he is promptly recognized and deified as a god-king instead of being crucified, whereby he imposes socialist utopian rule where bigotry, hypocrisy, cruelty, and greed are basically the only things that aren't allowed.

Persia, reaching the Indus Valley civilizations and the himalayas, has early contact with Buddhism which merges with Zoroastrianism and Christianity into a syncretic religion where do-no-harm and relieve-suffering are the supreme axioms.

When the Mongols reach them, they're welcomed with open arms, who then convert readily to this altruistic faith. As Buddhism spreads into China it carries this cosmopolitanism with it, and as they're never colonized by the British they face no humiliation or embarrassment, there's no need for a cultural revolution to undo the harms of capitalism, no need for a communist revolution because socialism has already spread from early Christianity on the wings of Persian tolerance.

Basically, world peace that will never be destabilized.

Even further back and kill an innocent fish.