We have a 13 year old resident cat and adopted a 9 year old female rescue cat just over two weeks ago. We have been following all the advice on introductions and, when we give them treats, play with them in each other’s presence etc it’s fine. However, at the end of the introductory session tonight the resident cat wandered off, and we didn’t realise at first when we went to put the new cat back in her room that he was in there. As soon as she saw him she ran for him and started a fight. We immediately separated them but are worried how to progress their introductions so they can ultimately live together and at least tolerate each other. The resident cat is not aggressive at all to the new cat, it’s the other way round. Both have lived with other cats but we don’t have much history about the new cat. Is there any advice anyone can give us?