New kitten and cat fight on sight every time

Thought we did everything correctly when introducing. Sniffing through the door, scent swapping, only feeding them treats when together, etc. Sometimes they're willing to sniff each other face to face first, but it always devolves into one of them slapping the other and turning into a whole thing. Older cat will run away and younger cat will pursue until she gets vocal and they get in a standoff. Not sure what to do