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.
late 14c., "relative, kinsman" (a sense now obsolete), from ally (v.); mid-15c. in the sense of "one united with another by treaty or league." Allies as the name of the nations aligned against the Central Powers in World War I is from 1914; as the nations aligned against Germany, Italy and Japan in World War II, from 1939.