Every copy editor and logophile will, from time to time, get roped into an argument about what the word decimate means and how it should be used. Etymological purists get upset when they find the word being used to indicate any general drastic reduction in the size of something. The exact size of the reduction, these purists argue, is inherent in the word itself: decimate stems from the Latin root decem, for the number ten, so the word must mean “to reduce in size by ten percent.”
Etymologically, this is true. It's also true that the word decimate originally referred to a drastic (but seemingly effective) disciplinary practice of the Roman army, in which a squad of unruly or subordinate soldiers would draw lots, and one in every ten soldiers would be executed.
But as we know, etymology tells us where a word came from, not what it means.
In seventeenth-century England, long after the massive armies of the Roman Empire were no more, the word decimation returned to describe a ten percent flat tax imposed by Oliver Cromwell. Decimation had become a deathless reduction in funds for the common people.
By the end of that century, people were using decimate to indicate any drastic destruction or reduction in size, remembering, as Merriam-Webster's Collegiate Dictionary puts it, “not the arithmetic of the Romans, but the ferocity of their methods.”
Still, it's worth remembering that decimate means a reduction in size, but not the complete elimination of something. For that, we have other words at our disposal.
Annihilate and obliterate
For all intents and purposes, annihilate and obliterate are synonymous. Both indicate complete destruction.
Annihilate comes from ad- “toward” + ne- “not” + hilum “trifle” and means “to reduce to nothingness.” Obliterate stems from ob- “over” + littera “letter” and originally meant “to make indecipherable or imperceptible by obscuring or wearing away” (MW11). But in common usage now, it simply means to destroy or erase completely.
Let your ear be your guide for deciding which word to use in a given situation.
If you relly wanted to differentiate annihilate and obliterate, though — and this is a personal guideline and not by any means a rule — you might let the violence or intenseness of the destruction be your guide, with annihilate being more complete or severe than obliterate. For example, you might obliterate a house of cards by knocking it over, but annihilate it by setting the cards on fire.
Keeping its etymology in mind, obliterate might be the better choice when thoughts or ideas are being destroyed, as in this mantra from Frank Herbert's sci-fi classic Dune:
I must not fear. Fear is the mind-killer. Fear is the little-death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain.
A more general and less complete destruction is indicated by devastate. From the Latin devastare, “to reduce (de-) to waste (-vastare),” devastation, unlike annihilation and obliteration, leaves something behind: chaos, disorder, or helplessness. People beset by devastation are left to pick up the pieces, but at least there are pieces left to be picked up.