"Great Regiment, the Battle Begins!" (大連隊、戦闘開始!, Dairentai, Sentō Kaishi!, Viz: The Main Regiment in Battle!!) is chapter 521 of the original Naruto manga.
Kabuto tells Tobi how he can stop Impure World Reincarnation, and then leaves. The shinobi army clashes with the White Zetsu Army. Kakashi's division arrives to help out the Surprise Attack Division. Zabuza and Haku ask about what had become of Naruto since their deaths. Kabuto uses Anko Mitarashi to strengthen the Impure World Reincarnation technique even more.