Russian-British Tit-for-Tat, Where Will It End?

Russia has expelled four British diplomats from Moscow just three days after the UK sent four Russians home. It's tit-for-tat over the poisoning death of a former Russian spy in London. Britain wants Russia to extradite Andrei Lugovoy to stand trial for the murder of Alexander Litvinenko but, today, Moscow upped the ante, saying it's now "impossible" to cooperate with Britain against terrorism. Michael Binyon covers foreign affairs for the Times of London.