How should Nato respond to any such challenge? The best answer would be to stamp on the first sparks from the first embers, says Jonathan Eyal, the head of security studies at the Royal United Services Institute. “If you have the first indication of self-proclaimed groups beginning to ask for autonomy or independence within one of the three countries, then you have to be involved immediately,” he says. “The key is to maintain an element of ambiguity: to say to Putin 'if you try it, then you might get a response which you never bargained for’. If you play some games in Latvia, then you might find 20,000 Nato troops arriving in the country.”