The truth is you cannot improve the breed, you can only alter it. "Improve" only makes sense if you have a specific trait in mind, like milk production or length of wool fibers.. In nature, you need a constellation of specific traits in certain combinations to produce a superior response to a constantly changing environment, and this is only in a statistical sense.

The fallacy of all eugenic fantasies is that they all imagine that genotypes are isolated and independent, and that they are associated with some characteristic the eugenicist values, like blond hair, blue eyes or white skin. Genetic superiority is not a function of purity, it only comes from diversity. In fact, genetic purity is a weakness, it is the mongrel species which are most able to adapt. This is why the agricultural specialists charged with preserving our grain stocks are not breeding super seeds, but instead collecting every wild variety they can in order to have as diverse a gene pool as possible available in case some pest or disease wipes out our grains. This is not what racists like to hear.

It's like trying to make a superior jet fighter by just giving it a bigger engine, but altering the engine has to be done at the expense of some other component. It doesn't work, what makes a superior fighter is an engine, armament, maneuverability, armour, sensors, ease of maintenance, economy of operation, i.e, thousands of interacting factors working in perfect harmony in just the right combination for the task at hand (which is shooting down your enemies' fighters). Even with simple engineering systems, like jet fighters, you don't even know if you have a winner until you send it up against other fighters and see how they compete in the real world.

Nature is good at this because it sacrifices most of its trials and lets natural selection do the work of finding the best combination. Eugenics will always fail because humans cannot afford the waste and losses that nature can tolerate. We cannot breed superior organisms because we don't know yet what a superior organism really is. All we can do is improve one trait at the expense of all others. And often, what we get is a creature so weak it is dependent on massive external support because it can no longer function independently in nature, like our racehorses and show dogs and miracle crops.

Social Darwinists like the Nazis always feel they have the right to enslave or exterminate what they feel are inferior races. They attacked East to wipe out the purebred Slavs, but they retreated West to surrender to the mongrelized Americans.