This question reminded me of a book I read in school in the mid-nineties. In today’s terms it was a young-adult novel, with an adolescent protagonist (?male), initially in a fairly-realist modern Earth setting, but with aliens quickly appearing in a major way. The aliens were from two highly advanced species — described as cat-like and dog-like, if I remember right — embroiled in a long-running war with each other. It eventually turns out that a previous visit from these species became the basis for biblical stories of angels and devils. I think the major plot arc was the protagonist and his allies preventing Earth/humanity from getting annihilated as collateral damage in the conflict.
I read it in English class in secondary school in the UK — I’d guess around 1996, certainly not later than 1998. By the style, I’d guess it was written in the 80s or 90s. I’d remembered the title as Prove Yourself a Hero, but that seems to be an entirely different book, not science fiction at all — possibly the book I’m thinking of is titled something similar, or possibly we just read both around the same time and I’ve confused them in memory.
