Nutshell: first third chronicles intergalactic duel between super-species, through proxies on earth, mostly; remainder involves virtually unrelated space opera contest between overachiever earthlings and trespassing pisceans.
Advertised as the first Lensman book, I’m not really seeing any of the items made famous by that series. Opening section indicates that super-species brought down Atlantis and Rome, and then are involved with the three world wars of the 20th century. No idea what all that has to do with the later space opera, which is more concerned with the fish-aliens, who are iron-maruaders. One super-species has a proxy in the solar system during the time, a faux pirate who acts the mad scientist.
Pisceans have some kind of evil ray gun that divests enemy fleets of their ferrous properties, including hemoglobin. Human military is therefore annihilated on first encounter. But captives on fish-ship are able to discern the principles of the ferrous-stealing raygun and deliver it to humans, who build a new “supership” in a few weeks (no shit!) and use that ship to defeat the fishes. W00t! Fucking uppity fish!