Victoria creates an army of newborn vampires and manages to evade Alice's vision by letting Riley choose her warriors, but Bella figures out her plan and tells Alice this on graduation day. That night at the graduation party, she receives a vision of the army's coming and the family prepares a battle strategy led by Jasper, while also forming allegiance with the wolves to increase the probability of a victory. During training sessions, she can use her visions to predict her opponent's movements. She fights the newborn army with the majority of the Cullens and the pack, but Jasper doesn't let her do anything out of fear for her safety and ends up getting mildly injured. Alice is highly upset when she sees a vision of Bella and Edward getting married in Las Vegas and confronts Bella about this. She tries to persuade her to let her arrange an official wedding for them to not much avail. After the newborn army's threat was eradicated, Bella makes a mature decision by giving in to Alice's wishes. Alice is visibly thrilled to be her maid of honor and getting to arrange everything for the wedding, with only veto-power on the guest list. Bella also tells her August 13th is the latest date for the wedding, which is exactly one month before Bella's birthday.

The events in Eclipse focus more on the newborn attacks caused by Victoria's creation of her newborn army in Seattle. Bella is introduced to the term "newborn" for the first time, and Jasper, who was involved with newborns in his early life as a vampire, explains them to Bella. This is how she learns about some of the issues she will have to deal with when she herself becomes a newborn. After following news coverage of several unexplained supernatural murders and disappearances, Jasper is the first to suspect that someone is creating an army of newborn vampires, but dismisses it for a while because he thinks he has read the signs wrong, until his suspicions are proven correct. Since Jasper has a great deal of past experience with the newborns, including combat knowledge, the Cullens turn to him for guidance in preparation for the upcoming battle against Victoria and her savage army. He helps train both the Cullens and the Quileute wolves by putting on sparring demonstrations, and pointing out the strengths and weaknesses of newborns for both groups. Jasper's military background helps him coordinate a strategy between the Cullens and Quileutes, which ends up working perfectly. Jasper, however, attempts to be everywhere at once during the battle, helping his family and the wolves fight while protecting Alice, and ends up getting bitten by a few newborns in the process. Jasper also tells Bella about his early life in the Confederate army, how he was turned, his involvement with Maria, Nettie and Lucy, and his role in helping to form Maria's newborn army, as well as how he met Alice.