The universe is so large that it's inconceivable that we'd be the only form of intelligent life in existence, but in the words of Physicist Enrico Fermi, "Where is everybody?" If there's lots of intelligent life out there, some far more advanced than we are, why isn't there any evidence that they exist? This contradiction between probability and evidence is known as the Fermi Paradox. There are potential explanations, but the implications of the Fermi paradox are often not very comforting and sometimes downright depressing.
In science fiction, first contact stories usually deal with this in some way, at least implicitly (if not explicitly). In The Three-Body Problem, Cixin Liu lays out one reason first contact should be carefully considered. Spoilers for all three novels follow! In that novel, a Chinese scientist, disheartened by her Communist upbringing (during the Culural Revolution, her father is killed, she is persecuted for reading a banned book, other family members joined the Red Guard, etc...) and general cruelty of humans, basically invites a nearby alien race to come and "purify the human race." The Trisolarans live in an inhospitable star system and the relative comfort of a planet like Earth is attractive to them, so they naturally begin invasion procedures. Interstellar travel being what it is, even for a civilization more advanced than we are, it will take their invasion fleet 400 years to reach earth. To clear the way for the invasion, the Trisolarans send and advance party of Sophons (basically a computer/AI embedded into a single proton in a handwavey but bravura sequence in the book) that will spy on the humans and also halt human scientific research and development by interrupting experiments and giving false results, etc...
The Three-Body Problem won the Hugo award a couple years ago, thanks in part to its absence from Puppy-related slates (and yet, being the type of story that Puppies seem to like). The follow up, The Dark Forest, picks up where the first book left off: Trisolaran fleet on its way, Sophons blocking technological progress, whatever shall humans do? After some speculation on the general impact such events would have on society and politics, the book settles into an examination of a human plan called the Wallfacer project. The UN selects four individuals and provides them with unlimited resources in order to devise a counterattack to the Trisolarans. However, thanks to the surveillance of the Sophons, these four men must keep their true plans secret. The Trisolaran response, carried out by human traitors in an organization called the ETO (Earth-Trisolaris Organization), is to designate four individuals in opposition, the Wallbreakers.
Due to the need to keep these plans secret, they all appear to be rather simplistic and silly on their face. However, as the novel progresses and the Wallbreakers study their opponents, the true nature of the plans come to light. Wallfacer Frederick Tyler, the former US Secretary of Defense, has a public plan to create a fleet of mosquito ships with kamikaze-like pilots that will swarm the attacking fleet and detonate nuclear bombs. His Wallbreaker reveals the true plan, which involves bringing a huge amount of water into space and using it to fuel a massive hydrogen bomb (this plan was never all that clear to me). Wallfacer Rey Diaz, famous for repelling a US invasion of Venezuela, has a similar public plan of creating huge nuclear bombs, but his true plan, to use the nuclear bombs to launch Mercury into the sun, thus destroying the entire system (including Earth), is exposed by his Wallbreaker. While this might have been an effective deterrent, it was revealed too early and the rest of humanity wasn't too keen on the plan. Wallfacer Bill Hines, a British neuroscientist, wants to use genetic modification to improve the human mind. His true plan is subverted by his Wallbreaker, who is also his wife. Details are a little unclear, but it turns out that the "Mental Seal" device he created actually instills defeatism in its users. Fortunately, the process was never fully adopted.
Finally, there's the most unlikely Wallfacer, Luo Ji. If this book could be said to have a protagonist, it would be him. He immediately refuses the honor, but his refusal is taken to be part of his plan. Resigned to his fate, he simply adopts a hedonistic lifestyle, finding an isolated home, drinking expensive wine, and using the UN as a dating service to find an attractive woman (who, for some reason, goes along with this?) Eventually, he reveals a public plan to transmit a "spell against the planets of star 187J3X1" into the universe. He says this will take at least one hundred years to work, but he predicts that his spell will be devastating. For their part, the Trisolarans seem the most afraid of Luo Ji, and rather than assign a Wallbreaker, they simply try to assassinate him. Luo Ji escapes barely, and manages to make his way into hibernation.
200 years later, he awakens to a changed world. Environmental degradation has lead to large underground excavations. Despite the Sophon block, technology has increased dramatically, and humanity now sports a fleet of thousands of spaceships. Observations of the Trisolaran fleet show trouble for our enemies, as the size of the fleet dwindles (presumably due to accidents or damage sustained during high speed travel). Humanity seems to regard the Wallfacer program a failure and is now seeking to establish diplomatic talks with the Trisolarans. Despite this, Luo Ji has to dodge a series of assassination attempts after he awakes, so clearly the Trisolarans are still scared of him.
All's well, right? Well, not so much. The arrival of the first Trisolaran probe results in a devastating attack on humanity's space capability (dubbed "The Battle of Darkness"). If such a tiny probe is so advanced, humanity has no chance against even a weakened Trisolaran fleet. That is, until Luo Ji's spell finally takes effect and star 187J3X1 is destroyed. His Wallfacer plan is thus finally revealed, and it relies on one of the more depressing explanations for the Fermi Paradox: While intelligent life may be plentiful in the universe, if you reveal your location, at least one of those civilizations will be both more advanced than you AND be a scary, predatory culture that will only see you as a potential threat. The strategy of such a civilization would be to preemptively strike any developing civilization before it can become a true threat. Luo Ji had sent out a message indicating that it came from star 187J3X1. This message was presumably received by lots of alien civilizations, but it eventually reached a more predatory species who simply destroyed the system. The title of the book comes from a metaphor: The universe is a dark forest where every civilization is a silent hunter. Any civilization that announces itself becomes a target.
Phew, that's a lot of plot, and believe it or not, I'm greatly simplifying here and leaving lots out. Like its predecessor, this book is stuffed with plot, ideas, and little thought experiments. This makes for interesting reading, and the overarching conflict is tense and exciting, but in execution it does feel a bit scattershot. The concept of the Wallfacer project is great, but it takes a bit too long to get at the hidden plans, and we spend a lot of time with characters who are closed off and focused on seemingly tangential plot points. It turns out that in deceiving the Sophons, the Wallfacers also have to deceive the reader, which is a great idea, but Liu only barely clears that bar, making this an entertaining read, but one that feels like it has a lot of filler. By design and for good reason, but filler nonetheless. That being said, I was surprised that it didn't manage to make the Hugo ballot last year. Then again, it's not like I read it back then or nominated it, so I don't have to look far for an explanation.
So finally we get to Death's End, the conclusion to Liu's trilogy and one of the nominees for this year's Hugo Awards. Naturally, this one starts with a segment set during the Fall of Constantinople. Without spoiling details, it ends millions of years in the future. Inbetween, we get some other approaches to the Trisolaran threat that parallel the Wallfacer project (a timeframe referred to as the "Crisis Era"), such as the Staircase Program (an attempt to send a lone human emissary to meet the Trisolaran fleet). We eventually get to the "Deterrence Era" in which Luo Ji is known as the Swordholder and deters the Trisolarans with mutually-assured destruction. But Luo Ji is getting old and must be replaced. His replacement is Cheng Xin, who worked on the Staircase project. Unfortunately, at the moment of transition, the Trisolarans immediately attack (using their probes and Sophons, etc...) and it's revealed that a new invasion fleet, capable of light speed, has set out from Trisolaris and will arrive in 4 years. Cheng Xin does not initiate the Dark Forest broadcast (because that would kill both civilizations) and the Trisolarans start colonization procedures, allowing humanity to collect itself in Australia (while the Trisolarans will take the rest of the planet). When the full implications of this emerge (the Trisolarans expect only about 50 million humans to survive), humanity gets all uppity and ends up broadcasting the location of Trisolaris into the Dark Forest, resulting in its quick destruction. It's only a matter of time before that same scary, predatory race intuits Earth as the origin of these broadcasts, so the Trisolaris fleet changes directions and flees into the galaxy. Humanity works on ways to counter the predatory race, either by hiding from it, escaping to interstellar space, or a few other tricks. Will they succeed?
Once again, what we have here is a novel that is overstuffed with ideas and thought experiments. Liu has a knack for naming things to evoke archetypal characteristics. Wallfacer/Wallbreaker, Staircase, Swordholder, and even the various Eras referred to throughout (Broadcast Era, Bunker Era, etc...); all of these lend a certain feeling of universality and scope that make this seem classical and enduring. High ambition, high stakes (that are actually earned), and a willingness to confront uncomfortable ideas and take them to their frightening but logical conclusions. I won't spoil the ending, but it's bittersweet at best, and existentially terrifying at worst. There's a reason that Fermi Paradox folks like to say "No news is good news" and this novel nails why that statement works.
Those ideas that evoke the fabled SF goal of Sense of Wonder are what make these books work. The more sociological and philosophical aspects of the story are a little less focused and successful, leading to some inconsistency in terms of characters and pacing that perhaps make the series too long and pull the books down a peg or two. I suspect some things are lost in translation here, but this is not meant as a slight on Ken Liu (who translated the first and third books in the series), just an acknowledgement that translations naturally produce, for example, awkward dialog and pacing. I'll put this on me too, as reading a book from another culture always presents challenges that I'll readily admit I'm not always equal to. However, most of my complaints are far outweighed by what this series gets right, and this will rank high on my Hugo ballot, though I don't know that it will unseat my current frontrunner (which remains Ninefox Gambit). This isn't quite the diamond-hard SF of Greg Egan or Peter Watts, but it's fully in the tradition of "literature of ideas" and even if some of those ideas don't land for me, it's definitely my kinda SF.