Military incursion (8)
I believe the answer is:
invasion
'military incursion' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for invasion that I've seen before include "Offensive" , "Encroachment, intrusion" , "Unwelcome encroachment on another country" , "Unwelcome intrusion" , "Hostile incursion" .)