Triplanetary

This is really a "prequel" written somewhat later, but in internal chronology it is the first in the Lensman series, and fills in some background. We are introduced to the benevolent Arisians, who are responsible for all life in this universe through a kind of panspermia. Then the evil Eddorians come in from another universe looking for worlds to rule, and the battle of the millennia ensues. The human race is one of the "Arisian" races, and the plan involves a breeding program over centuries leading up to something, and this book tells some of that story.

I read this as part of a collection "The Works of E.E. "Doc" Smith"

Although it feels dated in some aspects, it is one of the great classics and a must read for any sci-fi fan.