When my neighbors cat enters the backyard my two cats suddenly turn into a crime fighting duo, even the dog gets in on the action, it’s all paws on deck against the intruder.

The rest of the time they’re all vying for control of their piece of the house.

If my mom's dog starts shit with the younger cat, the older one will lose his shit and attack the dog.

I think that depends on how they view the house as their territory

when we had a lot of cats and the occasional intruder cat, "duels" between one of ours and one intruder were most common. meet on neutral territory, hurl insults, maybe there's an attack but maybe not. and only after sufficient insults.

this was a specifically male behavior, too. we got a young female cat, and the 2 older males were horrified when she came and jumped their opponent. she was very no holds barred, get off my lawn, don't mess with my guys. a different time, a different young female cat was equally willing to jump in swinging claws and skip the insults.

typically the other male cats would be hanging around, observing and jumping in only if things moved from neutral ground to their territory.

They do infact team up hell my cat army of 4 even protects their weaker friends I really don't know why my nabors cat continues to come into my lawn when he gets ganged up on by 3 of my 4 cats

When my roommate's dog chases my roommate's cat, my cat jumps in to defend him.

We had two cats who wouldn't be in the same room as each other without hissing, but they teamed up to trap a neighbour's cat under my car one night.