Star Wars spoiler warning:
The Battle of Endor, between forces of the Rebel Alliance and the Evil Empire, was undoubtably the great turning point in the interstellar war between those two forces.
Initially this battle was planned by the Alliance to be a guerrilla action to destroy the Second Death Star while it was still under construction in orbit of the forest moon of Endor. This plan counted on a ground force disabling the death star's shield generator found on the surface of the moon, so that a space-based strike force, led by Admiral Ackbar, could proceed into the station's infrastructure and destroy its main power generator directly. The theory was that with most of the Empire's forces spread throughout the galaxy in an attempt to engage the Alliance, that a concerted attack, with the station's shields down, would be enough to overpower the few Star Destroyers defending the facility (including the Super Star Destroyer Executor).
As with all great plans, this one did not survive the first moments of battle. The ground force successfully made it to the forest floor, using a captured imperial shuttle and access code to obtain clearance. However, this is where the plan began to fall apart. Firstly, Princess Leia became lost in the forest following a speeder bike chase with a number of imperial scout troopers, and was almost captured until she was rescued by an Ewok. Then, when Han, Luke, Chewbacca and the droids attempted to go looking for her, they fell into an ewok hunting trap and were almost cooked over open fires until C-3P0 convinced them not to do so (with a little assistance from Luke's force powers to give the illusion C-3P0 was a god).
During the night, Luke surrendered himself to Imperial forces, demanding to see Darth Vader in the hope of bringing him back from the dark side. He is taken from the planet's surface to the Death Star, where Vader is waiting to take him to the Evil Emperor. Together, they try to convert Luke to the dark side, attempting to feed on his anger and guilt, enticing him to take his lightsaber and attempt to strike the emperor down.
In the morning, the ewoks aided the rebel ground force by guiding them to a hidden entrance to the imperial shield generator facility. While the rebels were initially successful in entering the facility, they were quickly discovered and captured in an imperial counterattack. When all appeared to be lost, the ewoks made their return appearance, ambushing the imperial stormtroopers with masses of primitive weaponry, ranging from bows and arrows, slings, and even a number of elaborate log traps designed to destroy imperial AT-ST walkers.
Meanwhile, in orbit of the planet, the rebel fleet came out of hyperspace and began their attack upon the Death Star, unaware that the shields had not been taken down. Only at the last moment did Lando Calrissian, flying the Millennium Falcon, realise that the shields were still up, and aborted the attack run. From here, things went from bad to worse. Firstly, the entire imperial battle fleet arrived on the scene, having been hiding behind Endor, waiting in ambush for the attack. Secondly, the Death Star was further along in its construction than the rebels realised, permitting it to lash out with its superlaser, annihilating rebel capital ships in one shot. Desperate to evade the superlaser, the rebel fleet engaged the imperial capital ships at close range, obscuring a clear shot from the Death Star. During this battle, concentrated fire brought down the Executor's bridge shielding, enabling one A-Wing pilot to crash his craft into it, disabling the vessel and resulting in the empire's flagship plummeting into the surface of the Death Star.
Meanwhile, down on the surface, the rebel ground force gained the upper hand over the imperial forces, then tricked those inside the shield generator facility to come out, where they were quickly captured. This finally permitted them to destroy the shield generator, making the Death Star vulnerable to attack. Immediately upon this happening, several rebel craft, including Wedge Antilles' X-Wing, and the Millenium Falcon piloted by Lando Calrissian, entered the space station's superstructure, pursued by a number of TIE Fighters.
Back aboard the Death Star, Luke's rage grows at the sight of the alliance fleet being slaughtered by the Imperials. He finally responds to the Emperor's taunts, taking up his lightsaber and battling Darth Vader. Finally, Luke gains the upper hand, cutting off Vader's hand and knocking him to the ground. However, rather than striking a death blow, Luke turns away from the battle, refusing to fight further. Enraged, the emperor lashes out at Luke with lightning through the force. Then, as Luke is in throes of pain, Vader turns upon his former master, flinging the emperor down a huge pit into the bowels of the station.
Back in the space battle, after a determined chase, the rebel ships reached the centre of the facility, destroyed the power generators, and flee, pursued by gouts of flame as the station begins to die. Meanwhile, Darth Vader dies in Luke's arms, leaving Luke to carry his father's body to a shuttle and make his escape as the station dies around him.
With the destruction of the Death Star, and the death of their leader, the demoralised imperial fleet were routed, leaving the Rebel Alliance as clear winners. Following this battle, the Evil Empire never recovered its former glory, as the imperial command structure is fractured by officers trying to carve out their own empires. Indeed, in only a few years after the Battle of Endor, the Empire's capital of Coruscant fell to the rebels, never to be reclaimed.