late 13c., "to join in marriage" (transitive), from Old French alier "combine, unite," from a differentiated stem of aliier (from Latin alligare "bind to, tie to," from ad "to" (see ad-) + ligare "to bind, bind one thing to another, tie" (from PIE root *leig- "to tie, bind"). Meaning "to form an alliance, join, associate" is late 14c. Related: allied; allying.
c. 1300, "bond of marriage" (between ruling houses or noble families), from Old French aliance (12c., Modern French alliance) "alliance, bond; marriage, union," from aliier (Modern French allier) "combine, unite" (see ally (v.)).
General sense of "combination for a common object" is from mid-14c., as are those of "bond or treaty between rulers or nations, contracted by treaty" and "aggregate of persons allied." Unlike its synonyms, "rarely used of a combination for evil" [Century Dictionary]. Meaning "state of being allied or connected" is from 1670s. The Latin word was alligantia.
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Harper, D. (n.d.). Etymology of allied. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/allied