This is one of those books where even though it may not be the deepest book you've ever read and sometimes your suspension of disbelief wobbles, you just keep reading because you have to know what happens next, and the shift between multiple POVs actually works to increase the tension and invest you in the story. I believe this is what we refer to as a "page-turner."
In the beginning, Defenders is your basic alien invasion story. A race of telepathic giant starfish-like aliens called the Luyten land on Earth and in a long campaign of escalating attacks, eventually have humanity on the ropes and facing apparent extinction. Besides their superior technology, the Luytens' telepathy makes them almost impossible to defeat. When they begin wiping out entire cities, driving humans out and then slaughtering them with heat rays, the President of the United States, as Commander in Chief of the improbable Global Alliance that has formed to fight the Luyten, is preparing to offer mankind's surrender.
Then humanity unleashes their Hail Mary superweapon: a genetically-engineered race of giant three-legged supersoldiers who are immune to the Luytens' telepathy. The Defenders, in a matter of months, turn the tables on the Luyten, and soon it is the aliens who are surrendering.
Unfortunately, the scientists who created the Defenders didn't really think about what they'd do after the Luyten were defeated.
I liked this book a lot, mostly because it just kept ramping up the tension, but also because all of the characters — human, Luyten, and Defender — managed to be fascinating in their own ways. The Luyten initially appear to be genocidal monsters bent on humanity's extermination, yet through conversations some of the humans have with Five, a captured Luyten, it's evident that they are much more complex and their motives may not be so clear-cut. The Defenders, for their part, were created to be weapons against a specific enemy, and are left without a purpose once that enemy is defeated. Despite being based on humans, their psychology is nearly as alien as the Luytens', and unfortunately, they turn out to have just as much difficulty coexisting with mankind.
The multi-layered characters and the constant shifts in the fortunes of war (there being several wars during the course of the book) made this one of my best reads this year. It's almost a 5-star book, but it gets dinged for three things. First, the writing at times almost seemed aimed at juveniles, with very simple, repetitive sentences that made me wonder if the author got told in some writing workshop that long sentences and big words are bad. Second, there were several big suspensions of disbelief — one or two I could handle (like human scientists being able to whip up a genetically engineered race of giants and mass-produce them in factories around the world, in response to an alien invasion that is wiping out every country's infrastructure - or mankind then ceding Australia to those giants once the war is over) - but after a while the big ideas plus some of the smaller ones became so cinematic I just nodded along without buying any of it. Lastly, there were so many questions I really wanted answered. The Luyten spend years not communicating with humans at all, and then suddenly are rallying them as a species. They go from exterminating invaders to sympathetic and almost admirable, but there is never really an adequate explanation of why their initial attack on Earth was so bloodthirsty, with no attempt at more peaceful negotiations. And much is left unanswered in the epilogue.
Maybe some of the questions are meant to be unanswered - one of the crucial ones, echoed by the characters over and over, is whether one can ever truly trust the Other. Luytens mistrust humans, humans mistrust Luyten, both hate the Defenders who hated the Luyten and admired but felt inferior to humans, that inferiority complex turning to rage when they felt betrayed because humans reacted with paranoia to what seemed to be Defender aggression... multiple devastating global wars started because two races couldn't just talk to each other. Although this book is very much a comic bookish sort of epic, something that would look good on screen with a big enough special effects budget, these themes make it perhaps deeper than it was intended to be.
Thus, Defenders gets 4.5 stars - not a perfect book, and the writing could use some polish, but it's one of the best alien invasion stories I've read in a while.