co-belligerent (n.)

in Latin, the form of com- "together, with" in compounds with stems beginning in vowels, h-, and gn-; see com-. Taken in English from 17c. as a living prefix meaning "together, mutually, in common," and used promiscuously with native words (co-worker) and Latin-derived words not beginning with vowels (codependent), including some already having it (co-conspirator).

1570s, "waging war, engaged in hostilities," from Latin belligerantem (nominative belligerans), past participle of belligerare "to wage war," from bellum "war" (see bellicose) + gerere "to bear, to carry" (see gest). The noun meaning "party or nation at war" is from 1811. Related: Belligerently.