Ayn Rand has said that "evil" which she defined as anything that is "anti-life" or most specifically against mans life is "impotent".
However I can think of a clear example of "potent" evil overcoming "good".
Say, if a mad scientist invents a self-replicating, nano-machine that converts everything it touches (living and non-living) into copies of itself, this creates a situation where you'd have to ask two questions:
A) How does humanity defend itself against this class of threat (clearly designed to destroy humanity).
B) How is this example of "evil" not "potent" in achieving its goal?
This technology is powerful and (realistically) not far off. As our technology accelerates in the coming decades, the potential for great "evil" will become magnified.
How does Objectivism answer this?
Evil is not impotent when it is assisted by the good. The good too often gives evil vastly more power than it would ever have on its own, unaided by the good. That's the whole issue of "the sanction of the victim" which Ayn Rand concretizes in Atlas Shrugged.
It is the scientists who are not "mad" who can do the most damage. The ones who are "mad" don't accomplish much and can't; they are, indeed, impotent.
The question also asserts, without references or elaboration: "This technology is powerful and (realistically) not far off" -- referring to "a self-replicating, nano-machine that converts everything it touches (living and non-living) into copies of itself...." The questioner's description sounds very similar to a virus, not much different from any biological organism that survives as a parasite on other ogranisms. If the parasite is too virulent, it eventually destroys itself by running out of new victims to feed on. Other technologies, such as nuclear bombs, can cause a lot of damage, too. Knowledgeable individuals who are capable of assisting the destructive misuse of technology of any kind certainly should think twice about doing so, and perhaps join with those seeking to defend themselves against such attacks, if they value their own lives and well being as well as respecting the lives of others.
The original question and follow-up comments by the questioner rely on several key concepts, each of which needs close examination:
There is also a related concept to consider: appeasement.
The main premises of the question apparently are that (1) evil is destructiveness, and (2) the power to destroy is a form of being "potent."
In casual or ordinary usage, (2) may seem uncontroversial. To find out how Ayn Rand actually uses the term "impotent" (or impotence), I did some searching in the literature of Objectivism. I found a number of illuminating references, but the one that seems to provide the most definitive analysis of all the related aspects of the question is to be found in OPAR, Chapter 9, pp. 329-333. Page 329 introduces the principle that evil is impotent, along with a footnote reference to Atlas Shrugged. Here is the OPAR description:
Evil, for Objectivism, means the willful ignorance or defiance of reality. This has to mean: that which cannot deal with reality, that which is whim-ridden, context-dropping, self-contradictory. Evil is consistent in only one regard: its essence is consistently at war with all the values and virtues human life requires.Page 331 explains the "power" of evil:
Evil does have one power. It has not the power to create, to set positive goals and achieve them, but the power to destroy: to destroy itself and its victims. [...]Page 332 explains what happens to evil in a proper society:
In a proper society, however, evil is a marginal element. When men live by rational principles, the evil, so far as men can identify its presence, is ostracized and stopped. Under these conditions, even its power to destroy is largely nullified -- except in regard to the evildoer himself.This is followed by considerable further discussion and examples of "road-paving," culminating (on p. 333) in "Ayn Rand's historic identification ... the sanction of the victim."
In the rational society envisioned by Objectivism, the evil has no foothold on the living power of the good and no way to offer it torture as recompense. On the contrary, the evil is both damned and dammed, while the good is left free to achieve values and enjoy them.From these excerpts and others, note also that appeasers are certainly regarded by Objectivism as morally harmful, but the term "evil" tends to be used more (in Objectivism) to refer to explicit advocates and practitioners of anti-life moral principles than to appeasers who continue to produce the values which the altruists demand (and who may feel powerless to do otherwise). Here is an example (from "The Lessons of Vietnam" in VOR, p. 138):
I wondered, even in those years [Ayn Rand's adolescence], which is morally worse: evil -- or the appeasement of evil, the cowardly evasion that leaves an evil unnamed, unanswered and unchallenged. I was inclined to think that the second is worse, because it makes the first possible. I am certain of it today.
Objectivists do not believe that evil people or ideological movements are impotent. Indeed much of the course of human history has been marked by evil winning out over the good. When Rand points out the impotence of evil, she clearly does not claim or imply that evil people/movements cannot accomplish their ends, but rather that to the extent that they are efficacious they are relying parasitically on the good.
The fundamental "sin" according to Rand is evasion. A thoroughly evil person with no iota of good would be chronically evading reality, and thus would indeed be completely inefficacious. He would not live long either, since even eating one's meals requires some focus on reality. The point is that people we recognize as evil are really mixed cases--mixtures of evil and good. The good sustains the evil and allows it to have power that it could never attain on its own.
Consider the Mad scientist--where did he gain his knowledge of science? Clearly he had to devote time and effort to focusing on reality in order to gain knowledge. Such reality focused-rationality is good. It is only after the good has created the value of the knowledge that the evil can turn it to sinister ends.
Not only do evil people rely on their own pittance of good character to help them get by, they also rely on the good around them. Consider again the Mad Scientist--where does he get his funding for his research, his equipment, his food, his assistants, etc.? The funding must come from someone who produced it, i.e., from someone who has done good. The equipment had to have been invented by someone, and then produced by someone, i.e., from someone who has done good. His food had to have been grown, harvested, and distributed by people, i.e., from people who have done good. And so on. Everything the Mad Scientist relies on so that he can reach his evil ends are provided to him by the good. If the good removed this support, then the Scientist would not accomplish much.
Pointing out some crazy scheme whereby evil wins over the good does not refute the principle that evil is impotent. It merely shows how spectacularly otherwise good people can slit their own throats by supporting evil.