The Order of the Phoenix is the main protagonistic organization of the Harry Potter series. The group was founded by Albus Dumbledore in 1970 after Lord Voldemort sought to take over Great Britain and establish a magical dictatorship that purged muggle-borns and anyone else they deem is "stealing" magic from the wizards.
First Wizarding War
In 1970, the organization was founded by Albus Dumbledore during Lord Voldemort's first rise to power. The Order also sought to recruit many Aurors (wizarding police) from the Ministry of Magic to participate in secretive assaults against the Dark Lord and his followers.
A prophecy told by Sybil Trelawney stated that a boy who fit the description of both Harry Potter and Neville Longbottom would grow up to kill Lord Voldemort. When the Dark Lord hears of this he decides to go after Harry Potter. Order member Peter Pettigrew betrays the Potters and gives away the location of them to Voldemort. When he arrives at the Potter residence, Voldemort kills James Potter first when he attempts to attack him to protect Lily and Harry. When Voldemort goes upstair to finish off the mother and the baby, Lily throws herself in front of her son to save him which causes the Killing Curse to deflect off of baby Harry due to sacrificial protection charm fueled with the intense magic of love. This ends up destroying Voldemort's physical body.
After Voldemort's disappearance four of his followers, being Barty Croutch Jr and Rodolphus, Rabastian, and Bellatrix Lestrange, go looking for him. They eventually get a hold of two Aurors who were also members of the Order, Frank and Alice Longbottom, parents of Neville Longbottom, who they torture relentlessly with the Cruciatus Curse to obtain the whereabouts of their leader. The torture eventually causes Frank and Alice to lose their sanity and throws them into a vegetative state. This caused the four Death Eaters to be sent to Azkaban.
Second Wizarding War
In 1995, Voldemort returns and Harry Potter returns form the hedge maze with a dead Cedric Diggory. When Voldemort returned due to the potion made by Pettigrew, the potion contained Harry's blood which also contained the love charm that Harry's mother performed with her sacrifice. The sacrificial protection charm will end up living on in Voldemort's blood even when Harry turns 17. Not soon after, the Order starts back up when order members are notified. The new Order's headquarters is established in Sirius Black's childhood home.
The Ministry of Magic greatly opposed the claim pushed by Harry and the Order that Voldemort has returned until the Minister of Magic, Cornelius Fudge, sees the Dark Lord himself inside of the Department of Mysteries which causes the Ministry to eventually back up the Order in combatting Voldemort and his Death Eaters.
Voldemort lured Harry and many of his friends to the Department of Mysteries to obtain the prophecy that spoke of Voldemort's downfall. The group of students were ambused by twelve Death Eaters including Bellatrix Lestrange, who escaped from Azkaban, and Lucius Malfoy. The Order came to the rescue when many members, such as Sirius Black, Remus Lupin, and Nymphadora Tonks. During the battle the prophecy was destroyed, most of the Death Eaters were arrested, and Bellatrix killed Sirius Black by shooting a spell at him that caused him to fall into the veil. This battle marked the beginning of the Second Wizarding War.
The leader of the Order of the Phoenix, Albus Dumbledore was killed by Severus Snape due to Snape needing to gain Voldemort's full trust to be a successful double agent loyal to Dumbledore. Dumbledore came up with this before due to his life already being cut short by a horcrux curse that would soon envelop his entire body,
Under the leadership of Alastor Moody, the Order, excluding Harry himself manages to transport Harry successfully to the residence of the Weasley family despite being ambushed by Voldemort himself and many of his Death Eaters. Severus Snape, acting as a Death Eater, shoots a Sectumsempra spell but accidentally injures George Weasley by severing his left ear off. Moody and Harry's beloved pet, Hedwig, were the only two deaths that came out of this battle.
Voldemort managed to take over the Ministry of Magic which lead to the Weasleys to be forced into hiding.
The Order of the Phoenix fought the Death Eaters in Hogwarts when they attempted to take over the school and to stop Harry from destroying one of Voldemort's final horcruxes. Losses were plentiful on the side of the Order including Fred Weasley, Nymphadora Tonks, and Remus Lupin. Severus Snape was personally murdered in private by the Dark Lord to make sure that he had full control over the Elder Wand. Harry witnessing Snape's death allowed him to take a memory that Severus provided him with to the penesive and find out that he needed to die. Coincidentally, Voldemort, contacting Harry with the horcrux inside of him, tells him to meet him in the Forbidden Forest so he could kill him. He also offers that if Harry accepts this then he will spare everyone else. Harry goes to sacrifice himself and this ends up putting a love charm on the whole school which means Voldemort can not kill them at all. Harry survives the Killing Curse due to his mother's sacrifice being solidified through Voldemort coming back in 1995. He pretends to be dead and is brought back from the forest in the arms of Rubeus Hagrid. Harry ends up defeating Voldemort after their final duel when the Elder Wand rejects Voldemort as he is not the true owner and Harry's disarming charm deflects the killing curse onto Voldemort killing him.