1883, in reference to the Austrian military frontier in the Balkans; see de- + military + -ize. Demilitarized zone attested by 1921 (the Versailles Treaty uses neutralized zone).[demilitarize etymology, demilitarize origin, 英语词源]
1734, "to render neutral" (in a chemical sense), from French neutraliser (17c.), from neutral (see neutral (adj.)). Meaning "to counterbalance, to kill by opposing" is from 1795. Related: Neutralized; neutralizing.