Alliance since 1949 (4)
I believe the answer is:
nato
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">alliance since 1949</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for nato that I've seen before include "US/Europe alliance" , "International organisation for collective security" , "Military bloc" , "Western mil. alliance" , "Security agreement" .)