Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
insure, assure, ensure (vv.)
Americans frequently prefer insure when it comes actually to writing or buying insurance policies, whereas both insure and ensure have the generic sense of making certain of outcomes, as in Our careful planning will insure [ensure] our success. Assure often conveys that same general sense, but it can also be applied specifically to people: Her remarks seem to assure success is Standard, but Her remarks seem to assure us of success is much more commonly encountered in Standard English. See ASSURE.
