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, the 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 Lord Voldemort has returned. This prompts Hermione to suggest founding a student group where Harry would teach practical Defence Against the Dark Arts.

Oliver Phelps (October 21, 1749 – February 21, 1809) was born in Poquonock, Connecticut and moved to Suffield, Connecticut, where he apprenticed to a local merchant. He shortly thereafter became a tavern keeper in Granville, Massachusetts. During the Revolution he was Deputy Commissary of the Continental Army and served until the end of the war. He supplied troops and was commended by General George Washington.

During the Revolutionary War Phelps was Deputy Commissary of the Continental Army. He was introduced to Robert Morris, the great financier of Revolutionary times. He was a member of the Massachusetts House of Representatives from 1778 to 1780; and a member of the United States Constitutional Convention in 1779 and 1780. He was a member of the Massachusetts Senate in 1785, and of the Governor’s council in 1786. He was elected as a Democratic-Republican to the Eighth United States Congress, serving from March 4, 1803, to March 3, 1805, and ran unsuccessfully for Lieutenant Governor of New York in 1804 on the ticket headed by Aaron Burr.