The Battle of P3Y-229 was the first direct engagement between the forces of the Milky Way and those of the Ori, marking the beginning of the Great Enlightenment in the Milky Way with the battle itself taking place in 2379 with the fight itself ultimately ending with the Ori forces winning against the combined forces of the United Federation of Planets, Klingon Empire, Asgard, Tok'ra, Jaffa and Lucian Alliance and also marking the beginning of the Ori's attempt to claim the Milky Way Galaxy for themselves and convert all its species to the teachings of Origin.
Ever since the Ori made contact with the humans of the Milky Way, the resources of their home galaxy had been turned to construction of a fleet with which to convert them. Humans on several worlds were enlisted to build the ships, and conscripted as soldiers to serve in them. However, the distance between the Ori home galaxy and the Milky Way was apparently so great that travel between them was only possible via Stargate. This, plus the Ori's inability to interfere directly within the Milky Way for fear of the Ancients, limited them to sending Priors to coax local worlds to accept Origin.
Uncontent with this approach, the Ori sought to construct a much larger Supergate within the Milky Way, through which they could send their forces. The first attempt was thwarted, barely, by the efforts of Vala Mal Doran, who was able to interrupt the assembly process. Their second effort, however, succeeded.
Stargate Command was warned about the Supergate's presence by Vala, who had taken up residence in the Ori home galaxy; but she did not know its exact location. After an intensive search, a Free Jaffa vessel commanded by Cha'ra located the Supergate at P3Y-229; a combined fleet of ten Ha'tak from the Free Jaffa Nation, three Ha'tak motherships from the Lucian Alliance, 14 Klingon Vor'cha-class attack cruisers from the Klingon Empire, two Sovereign-class vessels the USS Enterprise-E and USS Intrepid-A, one Allegiance-class ship, the USS Odyssey, two Federation Daedalus-class battlecruisers, and one Asgard O'Neill-class ship were dispatched to try and prevent an Ori incursion. Typhuss took it upon himself to seek the aid of the Lucian Alliance as well, personally imploring Netan for assistance. Though Netan's first instinct was to kill him, Typhuss managed to convince him that cooperation was imperative.
By the time the USS Odyssey arrived on the scene, the Jaffa fleet had already attempted to destroy the gate, with no success.
Four Ori warships were sent through the Supergate, just after the Russian battlecruiser USS Korolev arrived. The Ori vessels transmitted only one cryptic message, a quotation from the Book of Origin. Dr. Daniel Jackson, on board the Korolev, transmitted a reply, also from the book, but to no apparent effect. The Ori vessels powered their weapons, prompting the allied fleet to open fire.
It became clear very quickly that the allied forces were severely outclassed. The Ori vessels carried a powerful beam weapon that could destroy a fully functional Ha'tak in a single shot, and even Asgard weapons proved ineffective against the Ori shield. Both the Odyssey and Korolev took heavy damage despite the protection of their Asgard shields. In the midst of the fighting, three Lucian Alliance Ha'taks arrived to flank the Ori ships, but fared no better.
Seeing that the engagement was going badly, Colonel Mitchell convinced Colonel Chekov to launch his ship's F-302 fighter-interceptors to provide cover fire. Whether this tactic would have had any success is unknown, as combined fire from two Ori ships overwhelmed the Korolev's defenses just as Mitchell was launching, destroying the ship.
Within minutes, the entire Milky Way fleet was slagged. Only the Odyssey, Enterprise, Intrepid, Odyssey NCC-78102 and Netan's Ha'tak survived relatively intact, and all were left crippled and badly damaged for hours afterward; the Asgard vessel's fate is unknown, although it is believed to have been destroyed.
The Ori stopped short of destroying the entire fleet, though they were certainly capable of doing so, presumably so that the survivors might spread the word of the swift and total Ori victory.
The battle of P3Y-229 would mark the only time the forces of the Milky Way were more or less unified against the Ori incursion.
In its wake, the Lucian Alliance, after arriving two new Ha'tak, immediately turned on the Federation, with Netan accusing them of luring his forces into a slaughter so as to weaken him. Only the arrival of a force of Jaffa vessels led by Bra'tac forced the Alliance to withdraw, forestalling another battle, after the Alliance vessels destroy Netan's crippled Ha'tak to prevent it from being captured.
The Ori vessels proceeded to begin forcibly converting worlds throughout the Milky Way, starting with Chulak.
As their victories continued to mount, they managed to drive a wedge between the Tau'ri and Jaffa leadership as well, a conflict that was only halted by the destruction of most of the Jaffa leadership with the fall of Dakara some months later.