At the end of Crossing the Line the entire bezeri population of Bezer'ej was destroyed by a nuclear bomb laced with cobalt. They were killed as an after effect of Lindsay Neville attempting to destroy the c'naatat organism that had infected Shan Frankland (human) and Aras (wess'har) and which would be a disaster for the human race back on Earth should any government get their hands on it. C'naatat grants the host near immortality, though at the cost of making the host different than the species it once was. Frankland can never go home to Earth because she would be a lab rat for centuries and Aras can never be a true part of Wess'har society. Also at the end of Crossing the Line, Shan Frankland died. One of the very few known (or believed) ways to kill an organism infected with c'naatat is the vacuum of space. Frankland deliberately stepped out of a ship without a suit so that Neville would not have the satisfaction of killing Frankland herself.
Now in The World Before the wess'har are gathering for a potential war against Earth. Since it was humans who were responsible for the genocide of the bezeri and that there is a line of responsibility back to Earth, the only thing that will save humanity is if they act in accordance with the wess'har notion of personal responsibility. The more people who try to cover for those responsible or make excuses, the worse the wess'har response will be. The Wess'har on Wess'ej have called their more aggressive kin from their home planet to help and these wess'har will take a stark response. Meanwhile Aras is trying to come to terms to the loss of Shan Frankland, his isan (a wess'har term for wife). Frankland was the only known individual to also be infected with c'naatat and he loved her. But, now Aras learns that Ade Bennett, a marine and a good man has been infected in the fight to capture Frankland (from the previous book) and a bond grows between them. Frankland is presumed dead because she was lost in space without a suit, but c'naatat is highly adaptable and anyone who read the first two volumes has to be asking the question: Is she really dead?
After the power of the first two volumes and the shocking end to Crossing the Line, The World Before has a lot to live up to. Karen Traviss has proven herself a talented novelist and one who can tell a brutal story and make it compelling like nothing else. But while The World Before has a lot going on, it feels more like a middle book than the middle book did. The novel serves to set up Matriarch far more than it does to advance a storyline here, and that's not a bad thing, but it does knock the novel a peg or two down below the first two volumes of the Wess'har Wars. What this means is that the writing is just as sharp, the emotions just as strong, but that the story doesn't have quite the same punch of narrative imperative that the first two did. There is resolution for the characters and so on a personal character scale, the novel completes a story arc, but it sets up a grander story arc that is not at all complete. To say that The World Before is a peg or two below City of Pearl or Crossing the Line only means it isn't quite as excellent as the previous novels but that it is also still far above nearly every other science fiction and fantasy novel I have read in years. Karen Traviss has set the bar awfully high for herself.