Wiktionary
comrade in arms
n. (context military English) A fellow combatant or soldier.
Usage examples of "comrade in arms".
The empire of Alexander the Great had broken up in the same manner, general battling general to hold the territory already possessed or to steal territory from a former comrade in arms.
He is out there, in command of one of our most advanced positions, and he is fighting and beating poor old Galiullin, his childhood friend and his comrade in arms in the German war.
Gandang moved quietly down the ranks, stopping occasionally to exchange a word with an old comrade in arms.