Possibility to limit period of military action (9)
I believe the answer is:
operation
'military action' is the definition.
(operation is a kind of military action)
'possibility to limit period of' is the wordplay.
'possibility' becomes 'option' (I've seen this before).
'to limit' indicates putting letters inside (inserted letters are restrained or limited).
'period of' becomes 'era' (I am not sure about this - if you are sure you should believe this answer much more).
'option' enclosing 'era' is 'OPERATION'.
Can you help me to learn more?
(Other definitions for operation that I've seen before include "battle" , "Action; process" , "running" , "Surgical or military procedure" , "Procedure; enterprise" .)