Nato Secretary General Anders Fogh Rasmussen has called the forthcoming meeting in Wales "one of the most important summits in the history of our alliance".
He said that member countries face a "dramatically changed" security situation, mentioning Ukraine and the threat from Islamic State.
Angus Raffle, 8, says his surprise meeting with the President at Stonehenge was "awesome". His mother says she "squealed like a teenager".
The language has been ramped up at this Nato summit, yet caution appears the order of the day after the mistakes of recent history.
The US President's motorcade pulled up at the school with his black stretch limousine bearing a Welsh flag for the first time in history.