Jenova War

The Jenova War is a war that occurs in the world of Final Fantasy VII. Referred to by this name in the 10th Anniversary Ultimania, the conflict constitutes most of the main plot of Final Fantasy VII, in which the forces of AVALANCHE lead by Cloud Strife attempt to stop Sephiroth from destroying the Planet.

Background
Approximately five years prior to the start of the game, Sephiroth vanished during the Nibelheim Incident, falling into the Lifestream while carrying Jenova's head. Unknown to the people of the Planet, Sephiroth exerted his willpower over Jenova, gaining the ability to control her cells, and began plotting to use the Black Materia to summon Meteor to strike the Planet. Sephiroth would absorb the Lifestream as it emerged to heal the wound caused by the impact, becoming a god-like being.

Cloud, the Shinra infantryman who threw Sephiroth in the Lifestream at Nibelheim, was used as a lab experiment by Professor Hojo before being broken out of captivity by Zack Fair and being taken to Midgar, where Zack entrusted Cloud with his Buster Sword and Cloud took on a false identity based on Zack's memories. Cloud presented himself in Midgar as a former SOLDIER member now working as a mercenary, and was hired by his childhood friend Tifa Lockhart to assist AVALANCHE in their insurgency against Shinra.

The War
When Cloud, Tifa, AVALANCHE's leader, Barret Wallace, and the Cetra Aerith Gainsborough, are captured by Shinra, Sephiroth puts his plan into action. The remains of Jenova kept in Shinra Headquarters take on Sephiroth's form and kill many of the employees, and in the chaos the prisoner cells are opened. Seeing Sephiroth's Masamune near the corpse of President Shinra, Cloud realizes Sephiroth is alive, and AVALANCHE turns their efforts to investigating his return. AVALANCHE pursues Sephiroth across the Planet, Sephiroth often appearing before them via Jenova and unleashing mutated monsters formed from Jenova against them.

At the Temple of the Ancients Cloud claims the Black Materia, but Sephiroth exerts mental control over him via the Jenova cells in Cloud's body, and seizes the Black Materia from him. Aeris, possessing the White Materia that can summon Holy to protect the Planet from Meteor, leaves to head to the Forgotten Capital and pray for Holy. AVALANCHE follows her, and in the altar under the city, Sephiroth impales Aerith after Cloud narrowly avoids Sephiroth's mental commands to carry out the deed himself. With Aerith dead, Holy apparently inactive, and Cloud realizing the influence Sephiroth holds over him, the party is discouraged but continues northward.

At the North Crater AVALANCHE sees Jenova as Sephiroth slaughter the Sephiroth Clones and sending them in the Lifestream to reclaim their Jenova cells. After Jenova's main body is killed and the Black Materia reclaimed, AVALANCHE finds Sephiroth's true body in crystallized Mako. Sephiroth manifests before them and reveals the truth of Cloud's assumed identity. This mentally shatters Cloud's mind, and believing himself a puppet to Sephiroth's will, he hands the Black Materia to him, and Meteor is summoned.

Shinra executes the first step in their Great Sephiroth Plan, a planned military offensive against Sephiroth. Shinra uses the Sister Ray cannon to pierce the energy barrier Sephiroth had erected over the crater allowing Cloud's party to descend into the Northern Cave to find Sephiroth, having learned that Aeris succeeded in calling Holy, but Sephiroth is containing it. In the depths of the cave the party kills Sephiroth, allowing Holy to move, and with the aid of the Lifestream commanded by Aeris's spirit, Meteor's impact over Midgar is halted, and Meteor is destroyed.

Aftermath
With the destruction of Sephiroth's physical body, he is once again cast into the Lifestream, continuing to exist in spiritual form. Jenova is nearly entirely destroyed, a few clusters of her cells surviving aside from those contained in experiments of the Jenova Project. Jenova's cells continue to endure in the Lifestream, resulting in the fatal plague Geostigma that culminates two years later when Sephiroth attempts to use Geostigma as part of another plan to resurrect himself.

Midgar is destroyed due to being at ground zero of the conflict between Meteor, Holy and the Lifestream, but through the efforts of Yuffie Kisaragi, Vincent Valentine, and the Turks, many residents evacuate in time. The survivors found a new city on the outskirts of Midgar's ruins, Edge.

The Lifestream's flow over the Planet shifted after Meteor's near-impact, causing Mako Reactors to no longer produce power and feeling indebted to the Lifestream and Holy for sparing them, the people of the Planet drastically reduce their usage of Mako and Materia, products refined from the Lifestream by Shinra. Shinra, their leadership fractured due to the deaths of most of their executives in the war, survives as a company but in a radically altered role.