E. E. Smith 
Triplanetary [EPUB ebook] 

From the atomic age in Atlantis to a world remote in space and time, two incredible ancient races, the Arisians and the Eddorians, are in the midst of an interstellar war with Earth as the prize. The Arisians, using advanced mental technology, have foreseen the invasion of their galaxy by the corrupt and evil Eddorians, so they begin a breeding program on every planet in their universe. Their goal…to produce super warriors who can hold off the invading Eddorians. (Goodreads)