NATO triennially reassembled involving many different countries (15)

I believe the answer is:

internationally

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'involving many different countries' is the definition.
I can't judge whether this definition defines the answer.

'nato triennially reassembled' is the wordplay.
'reassembled' indicates anagramming the letters.
'nato'+'triennially'='natotriennially'
'natotriennially' is an anagram of 'INTERNATIONALLY'.

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(Other definitions for internationally that I've seen before include "in every country" , "around the world" .)

I've seen this clue in The Telegraph.

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