The Prime Minister has told Nato delegates that they face "new and evolving dangers" in a statement opening the second day of the summit.
He also accused Russia of "ripping up the rulebook".
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.