"We 'Brits' referred to the Americans as Yanks during the war but it was not an offensive offering ..."

Hmmm ... not so sure about that. Back in the 80s (I think it was) I saw a film about American servicemen in North West England during World War II. The next morning I had a meeting with an American woman and before the meeting started we made small talk and I mentioned that I had been to the cinema to see a film called, 'Yanks'. As soon as I said it I saw that I had made a mistake and that she was somewhat offended. I could have lied about the film, I suppose, but then it might have been a film that she knew well and she could have quizzed me about it in detail and ... well, even more embarrassment!