"Declaration of War" (宣戦, Sensen) is chapter 467 of the original Naruto manga.
Sakura departs from Konoha with Sai, Kiba, and Lee, off to find and speak with Naruto. In the Land of Iron, Tobi sends Sasuke and Karin away. He then tells the Kage that Akatsuki seeks the tailed beasts for the good of the world. The beasts were created centuries ago when the Sage of the Six Paths split up the immense power of the Ten-Tails. If Tobi recombines the tailed beasts into the original Ten-Tails and seals the Ten-Tails within his body, he will be able to place an illusion over the entire world and end all wars. He asks the Kage to help him achieve this goal by giving him Naruto and Killer B (in doing so revealing that B was never captured). They refuse, so he declares the start of the Fourth Shinobi World War.