When Irina from the Denali coven comes to visit, she sees Renesmee from a distance and mistakenly believes she's an immortal child - a child turned vampire. That's illegal so she reports it to the Volturi, who come to Forks to execute the Cullens for the crime. Aro also has been waiting for an excuse to attack because he wants to use the opportunity to steal Alice for his Guard. The Cullens' friends gather as witnesses to give evidence that Renesmee's different. Some of them agree to fight with them at need. It's also revealed that the Volturi are corrupt, they've used similar excuses in the past to attack other covens whose members they want, another reason to fight.
However - SPOILER...........................
In the end, it doesn't come to a fight. The Volturi walk away after all their arguments are countered, because Bella's shield makes the outcome of the battle so close and unpredictable they don't want to risk it.
