<P> In Harry Potter and the Order of the Phoenix, the new Defence Against the Dark Arts professor, Dolores Umbridge, chooses to teach only the basic theoretical principles of the subject in her classes instead of practical applications, due to Minister for Magic Cornelius Fudge's erroneous fear that Albus Dumbledore is preparing to assemble a student army to overthrow him . This theory - only approach is widely unpopular among the students, especially those characters like Harry, who are in their fifth year and have to take their O.W.L. exams on the subject later in the year . Harry also believes that lack of practical experience makes them more vulnerable to Lord Voldemort's forces, though the Ministry staunchly refuses to accept that Lord Voldemort has returned . This prompts Hermione to suggest founding a student group where Harry would teach practical Defence Against the Dark Arts . </P> <P> After Dolores Umbridge learns about the project, she bans all unapproved student organisations, so meetings are secretly held in the Room of Requirement and announced to members through the use of enchanted fake Galleons created by Hermione . Cho Chang suggests the "Defence Association", shortened to "D.A.", as the official name for the group, but Ginny Weasley's suggestion of "Dumbledore's Army", to mock the Ministry's paranoia and to show the group's loyalty to Dumbledore, is chosen . </P> <P> When Cho's friend Marietta Edgecombe betrays the group to Umbridge (Cho herself while under the influence of the truth potion Veritaserum in the film), Marietta is cursed with pimples on her face as a result of Hermione's casting a spell on the D.A. membership list . Later on, to prevent Harry's expulsion and the incrimination of other members, Dumbledore claims responsibility for organising the group, then escapes when Ministry officials attempt to arrest him . Though the D.A. stops meeting following these events, three members--Ginny, Neville and Luna--join Harry, Ron and Hermione in the battle in the Department of Mysteries towards the end of the fifth book . In Harry Potter and the Half - Blood Prince, Neville and Luna are distraught due to the fact that the D.A. no longer exists . When Hogwarts is invaded by Death Eaters, they are among the members who join the Order of the Phoenix in the ensuing battle . </P> <P> In Harry Potter and the Deathly Hallows, with Harry, Ron and Hermione absent from Hogwarts, Neville, Ginny and Luna begin a covert rebellion against new headmaster Severus Snape by reactivating the D.A. However, Luna is abducted at Christmas and Ginny leaves during Easter, leaving Neville for a few weeks as the D.A.'s leader . Neville tells Harry that most of the D.A.'s activity ceased shortly after Michael Corner was tortured by Death Eater siblings Alecto and Amycus Carrow for trying to rescue a first - year boy from imprisonment . The group thereafter hide from the Death Eaters in the Room of Requirement, using a secret passage to the Hog's Head to find food . The D.A. believed that if Harry returned he'd lead them in a revolution against Snape and the Carrows and are disappointed when he initially refuses to let them help . Harry does lead an impromptu revolt, but only himself, Luna and Professors McGonagall, Sprout, Flitwick and Slughorn take part in it, with Harry and Luna taking out the Carrows, and the professors (rallied by McGonagall and Harry) driving off Snape . In the book's climax, the D.A. (alongside the Order of the Phoenix) plays an important role in the Battle of Hogwarts, giving Harry enough time to find the remaining Horcruxes . </P>

Who told umbridge about dumbledore's army in the book