Your examples are useful as proof that despite self-proclaimed efficiency of eugenics strategy to boost society to the next level, all the socities which adopted this concept still failed miserably. Sparta, Hun and Mongolian empires, Nazi Germany... They claimed superiority, tried to enslave neighboors and even the world, made the world fear their military skills and toughness, spread terror and chaos everywhere. That was the legacy they left.
We don't have a positive conception of them, because solely intelligence without mercy, love, compassion means nothing. The mechanical human being science tries to artificially build is doomed to failure.
Of course everyone is here to evolve and improve themselves as individuals, but it doesn't mean those who are on an upper level can eliminate or nuke the ones below. Theoretically, we are here to help each other towards evolution (in every aspects and spheres of our existence), and eugenics go completely on the opposite route.
Nazi Germany built a cannon so heavy it needed its own rail road just to move and not cause an earthquake each time it fired. They fought the entire world for 10 years, and the kill/death ratio was in their favor, and I think this is enough proof of their success. Also, if you read the unbiased history, you will realize that Germany was attacked first and Hitler was seeking peace in the first days of the war, maybe not with Poland but surely with the UK and the Soviets.
The real evidence of success is really in our modern day, as Germany is one of the most successful nations because of that one single decade of using this ancient science.
The Huns and Mongolians fought to gain new lands as the tundra had nothing to offer, also to gain more females, and they had a very successful reign. The Mongolians killed 10% of humanity before they were finished, and they were finished because they were stupid. They had the strength but not the genius, and even Genghis Khan was semi-literate.
As for the Spartans compared to the rest of the Greeks, they were the most successful nation not only militarily but also in socioeconomics; no beggars and no crime because those are the doings of a weak or cowards individuals, and those were slaves who weren't allowed to have children.
So Mongolians and Spartans didn't leave any significant legacy because they didn't use eugenics, depending on high IQ but rather on strength, unlike Nazi Germany, which left a huge legacy in rocket and airplane engineering, chemistry, mechanical engineering and much more.
The apple doesn't fall far from the tree, and 90% of the kids will be just like their fathers. So imagine if every criminal, stupid, and weak person were sterilized and only the smart and strong were left. What would your country look like in the future?
Control of eugenics doesn't work because the genetic crap-shoot has way more variables than can be taken into account.
Actually this is true.
Eugenics isn't 100% effective, but still it can deliver at least 50% expected results.
Although I had never heard of eugenics before. You were the first to hear and know about eugenics. Thanks OP for informing about the new topic.
After listening to your examples I think applying eugenics to any race would be negative. As you say Mongolia and Sparta practiced eugenics and they were successful in this practice. I think it's a downside for people with high IQs and strength as a result of its application. If you consider the current times, Mongolia is not doing very well at the moment. Being strong and having a high IQ naturally leads to jealousy and hatred, which is enough to divide a nation. Considering these issues, I think that eugenics should not be applied at all.
You didn't hear of it because it's the most controversial issue to ever exist, and whenever you talk about it, people will start discussing the funniest joke in history, which is "Human Rights."
Mongolians are a poor and weak nation today because they stopped the natural sequences. Each Mongolian soldier had at least four women of different races. Whenever they conquered a city, they brought the women back to Mongolia as sex slaves, to the point that foreign females outnumbered Mongolian females. Believe it or not, but 0.5% of the humans on this earth have Genghis Khan's DNA. Not to mention, the rest of the Mongolians and modern-day Mongolians are not related to the ancient Mongolians at all, as they are a mixed population from every nation that has been conquered by them. They lost the superior DNA that was created over thousands of years through suffering and strength; that's why they are a pathetic nation today.