We had a situation occur in a game where my opponent had a creature with first strike, and I had one that didn't.
They attacked and I blocked, first strike creature did it's damage which didn't kill my creature, then before I was to apply damage my opponent tried to cast an instant which would have killed my creature.
Is this legal?
If the instant hadn't resolved I'd have killed my opponent's creature, but because we ruled that it did resolve before my creature did combat damage my creature died and her creature lived to fight another round.