Join military group, European (5)
I believe the answer is:
unite
'join' is the definition.
(synonyms)
'military group european' is the wordplay.
'military group' becomes 'unit' (group of soldiers).
'european' becomes 'E' (abbreviation e.g. EU).
'unit'+'e'='UNITE'
(Other definitions for unite that I've seen before include "Make one" , "Group of workers" , "Gather together as one" , "Couple" , "fuse" .)