Unusual to set arm of our military against another (4)
I believe the answer is:
rare
This could be a double definition.
'unusual' is the first definition.
(rare can mean unusual or odd)
'another' is the second definition.
(thesaurus)
'to set arm of our military against' is the link.
This may not be right. Some or all of it may belong to one of the definitions.
(Other definitions for rare that I've seen before include "little done?" , "Excellent; uncommon" , "Scarcely seen" , "Infrequent like underdone meat" , "Might steak be" .)