Great Legend War

The Great Legend War (レジェンド大戦, Rejendo Taisen) is the first invasion of Earth by the Empire of Zangyack which united the first 34 Super Sentai teams.

Beginnings
The Great Legend War was the greatest crisis that Earth had ever faced. The overwhelming forces of the Zangyack Empire which seeks to rule the entire universe, was on a campaign of destruction driving people into the very depths of fear. But the Earth possessed forces that could stand against this abominable evil: the defenders of love, dreams and peace. They are the thirty-four Super Sentai who dedicated themselves to protecting the happiness of the people.

From above, countless Zangyack Battleships bombarded cities while hundreds of Gormin troops led by their Sugormin commanders fought against opposition. How long the war lasted until the Super Sentai sacrificed their powers is unknown, though it is known that before the final confrontation, there was a massive battle where over 100 robots/giants fought against the Zangyack fleet. Many of the robots were destroyed or killed during this battle.

Final Battle
"Everyone! We must combine the power of all Super Sentai and defend the Earth!"

- AkaRanger tells the 34 Super Sentai to sacrifce their power

Seemingly shortly after the mecha battle, the Goseigers were outnumbered by the Space Empire Zangyack after Gosei Great was brutally defeated. They were joined by AkaRanger and Big One, who told the Goseigers to join them in assembling all of the Super Sentai Groups to defeat the invaders. DekaMaster, DekaSwan, Black Knight, Princess Shinken Red, Wolzard Fire, MagiMother, Zubaan, Signalman, Rio and Mele appeared to clear a path for the heroes. All 34 Super Sentai groups converged and toppled whole squadrons of Gormin Sailors and Sugormin Non-Commissioned Officers, but were overwhelmed by the Zangyack battleships.

AkaRanger gave the order for all of the groups to combine their powers into one attack. Wolzard Fire decided, along with the extra rangers and heroes to infuse their power as well. The resulting blast wiped out the entire Zangyack invasion fleet. Consequently, all of the Super Sentai warriors lost their powers, which were dispersed all over the universe in the form of Ranger Keys. This was observed by AkaRed, a hero who first appeared five years ago. On Earth, the heroes awoke to discover that they had no powers, but Alata (Gosei Red) believed they still can protect the planet without their powers as the teams went their separate ways.

Super Sentai
The participants were all of the Super Sentai and extra heroes, except for the Gokaigers and AkaRed.

Zangyack

 * Zangyack Battleships (Hundreds)
 * Sugormin (Hundreds)
 * Gormin (Thousands)

Trivia

 * "Taisen" can mean both "War" and "Great War".
 * The Great Legend War is depicted in episode 1 of Gokaiger, The Space Pirates Appear. The 35th anniversary movie, Gokaiger Goseiger Super Sentai 199 Hero Great Battle, shows parts of the Legend War that weren't seen in the first episode.
 * During the Great Legend War, many similar Super Sentai teams were seen working together to fight off the Zangyack Gormin. For example, in the movie, the Bakuryuu Sentai Abaranger team was seen working with their dinosaur-based predecessors Kyoryu Sentai Zyuranger while in both versions, Riki Honoo and Kyousuke Jinnai were seen fending off Zgormin, Sasuke and Yousuke Shiina and their teams were seen in both versions as well.
 * For the Great Legend War, they used archived voices/fighting grunts for those who weren't seen out of thier Ranger forms and weren't in the movie.
 * Dead heroes like Rio, Mele, Burai etc. were brought back to life to fight in the Legend War. Who brought them back isn't clear. Apparently Rio and Mele being killed in the series and then revived to fight Long in Go-onger vs. Gekiranger is mentioned in the 199 Hero Movie pamphlet as something that happened, so this adds a bit more evidence that Gokaiger incorporates everything as "canon."
 * The Gokaiger tribute to Jetman, Wings are Eternal, also suggests that the supernatural beings of this universe can permit those who have died to go down to Earth to conduct important business, as was the case of Gai Yuki. With the dead Sentai warriors needed to fight the Zangyack, they may have been given special permission to go down to Earth to join their teammates in the war until its completion, upon which they will return to their deceased state. This also fits with the justification for having "Burai" back in the Zyu2 footage.
 * According to the pamphlet, there was a massive battle that happened prior to the main battle in the Legend War where over 100 robots/giants fought against the Zangyack fleet. Many of the robots were destroyed or killed during this battle.
 * This could explain why Gunmajin and Datas do not appear.
 * Kakuranger's mecha were confirmed destroyed in Gokaiger episode 45, when Gai explains to Ninjaman why he wasn't released from his jar at the end of a ten year punishment.
 * Two other series have had similar battles: In Kamen Rider Decade (where it was done first), it was the Rider War, which saw various Riders battle and fall to the titular hero. In the anime movie Pretty Cure All-Stars DX 3, the heroines sacrifice their powers and companions to stop a massive menace from overtaking the Earth. Note that both the Decade and Pretty Cure examples were undone in some way at the end.
 * For some reason, both TyrannoRanger with his armour on and DragonRanger with his armour on where seen fighting together.