Weasley twins

Dumbledore's Army

Dumbledore's Army members as seen in Harry Potter and the Order of the Phoenix
Universe	Harry Potter
Founded	Harry Potter and the Order of the Phoenix
Location	Room of Requirement
Leader	Harry Potter (Harry Potter and the Order of the Phoenix) Neville Longbottom (Harry Potter and the Deathly Hallows)
Purpose	To learn and practice magical methods of self-defence Resistance against Death Eater activities in Hogwarts
Affiliations	Order of the Phoenix
Enemies	Lord Voldemort Death Eaters Dolores Umbridge Ministry of Magic Inquisitorial Squad

Dumbledore's Army (or D.A. for short) is a fictional student organisation in J. K. Rowling's Harry Potter series that is founded by the main characters, Harry Potter, Ron Weasley and Hermione Granger, to stand up against the regime of Hogwarts High Inquisitor Dolores Umbridge, as well as to learn practical Defence Against the Dark Arts. It was founded in the fifth book, Harry Potter and the Order of the Phoenix.

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.

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.