Perhaps I'm the one that's being a bit thick, but my impression previously was that North Korea's claim that its nuclear developments were a response to the threat of aggression from the US looked like a flimsy and see-through excuse. Consequently it seems a bit of a dim diplomatic/PR move for the US to lend credibility to this claim by deploying new Patriot missiles in South Korea. Can someone explain why this is the right time to be doing that?