There was that cat in the news a few years back who drove off that dog that was attacking and dragging a little boy in that family.
https://www.youtube.com/watch?v=EEa6jZv-Khc
https://www.youtube.com/watch?v=LSG_wBiTEE8
https://en.wikipedia.org/wiki/Tara_(cat)
On May 13, 2014, Jeremy Triantafilo, a four-year-old boy, was riding his bicycle in his family's driveway in Bakersfield, California when Scrappy, a neighbor's eight-month-old Labrador-Chow mix cross, came from behind and bit his leg.[9] As the dog began dragging Jeremy down his driveway, Tara, who the family states was very attached to Jeremy, tackled the dog and chased him away before returning to Jeremy's side to check on him.
Jeremy needed ten stitches in his left calf following the attack. He quickly recovered and was thankful for Tara's actions calling her "my hero".[10]
If mean, if I were a cat -- smaller than the dog in question, and physically less-able to take on larger animals than a dog anyway -- and the dog was already doing a number on a human, that's not a fight I'd casually jump into. And while there are a few social cat species, like lions, I don't think that the wildcat ancestor of the housecat is a social animal, so it's probably not really geared up to be helping out other members of a pride or anything.
kagis
Yeah, it's solitary:
https://synapsida.blogspot.com/2020/03/small-cats-domestic-cats-closest.html
Among these three species, the one thought to be closest of all to the domestic animal is the sand cat (Felis margarita). This split off from the line leading to the wildcats and the Chinese mountain cat around 2.5 million years ago, just before the Ice Ages got going, while the other species (or their immediate ancestors) seem to have been around since the Late Pliocene 3 to 3.5 million years ago.
https://en.wikipedia.org/wiki/Sand_cat
The sand cat is solitary, except during the mating season and when a female has kittens.
That's a thought, though I'd also point out that this might involve international law, and there might be different doctrines involved in international law.
Also, international law on involvement in warfare is fluid. I remember reading an article pointing out that if you go back, to, say, the pre-World War era, the obligations on non-involved parties were generally held to be much stricter -- like, doing something like having preferential arms export policy to one party would be considered involvement in a conflict. When Switzerland, earlier, refused to export Gepard ammunition to Ukraine, that's not really in line with the present norm, where countries often do provide arms to countries and consider that to be separate from being directly involved, but it does conform to historical rules on neutrality.
kagis
Not the article I was thinking of, but this is some related discussion:
https://crsreports.congress.gov/product/pdf/LSB/LSB10735/3