Dumbledore's Army (the D.A. for short) is a student organisation in J. K. Rowling's Harry Potter series that is founded by the main characters, Harry Potter, Ron Weasley and Hermione Granger, in order to stand up against the regime of Hogwarts High Inquisitor Dolores Umbridge, as well as to learn practical Defence Against the Dark Arts.
In Harry Potter and the Order of the Phoenix, new Defence Against the Dark Arts professor Dolores Umbridge chooses to only teach 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 in order 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 Voldemort has returned. This prompts Hermione to suggest founding a student group where Harry would teach practical Defence Against the Dark Arts.
Hermione and Ron recruit students from Gryffindor, Hufflepuff, and Ravenclaw who all meet with Harry, Ron, and Hermione at the Hog's Head inn to discuss founding the group. When Umbridge learns about the meeting, she bans all unapproved student organisations, and meetings are secretly held in the Room of Requirement at least once a week for several months. Members are told when to meet through engravings on enchanted fake Galleon coins 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, in order to mock the Ministry's paranoia and to show the group's loyalty to Dumbledore, is chosen.
Cho's friend Marietta Edgecombe betrays the group to Umbridge, and 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.
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