"one who is mutually at war" (as distinguished from an ally), 1813, a word from the Napoleonic wars, from co- + belligerent. As an adjective, "carrying on war in conjunction with another power," from 1828.
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.
<a href="https://www.etymonline.com/word/co-belligerent">Etymology of co-belligerent by etymonline</a>
Harper, D. (n.d.). Etymology of co-belligerent. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/co-belligerent