The Battle for the Island was the final battle of The War, between Jack Shephard, the protector of the Island, and the Man in Black, who intended to destroy the island and escape it, killing everyone he left behind. The battle ultimately concluded when Jack and Kate killed the Man in Black, allowing Jack to save the Island from sinking.
Thousands of years before the present day, Jacob was given the job of island protector by his adoptive Mother. After Jacob's brother, the Man in Black, killed their mother, the enraged Jacob began to beat him heavily then he threw him into the Heart of the Island, causing his physical body to be killed and his spirit to live on as the Smoke Monster. ("Across the Sea") Jacob then set up a system of "candidates" to eventually replace him, while keeping the Man in Black trapped on the island. The Man in Black would have to kill Jacob and all his candidates to escape.
After taking the form of John Locke and using Ben to kill Jacob, the Man in Black set about manipulating the candidates. ("The Incident, Part 2") Unable to directly kill them himself, he was forced to use other methods. He placed a bomb in Jack's backpack after tricking him and the other candidates into boarding Widmore's submarine. Sayid, a candidate, sacrificed himself to get the bomb away from the others. Sun and Jin, one of whom was a candidate, drowned together on the sub as it sank. The three remaining candidates, Jack, Sawyer, and Hurley, along with Kate, set out to kill the Man in Black. ("The Candidate")
The Man in Black decided to use Desmond to destroy the island. Having been exposed to massive amounts of electromagnetism, Desmond was immune to the deadly effects of the Heart and could enter it. The Man in Black and Ben went to the well he had thrown Desmond down, but he was nowhere to be found. Meanwhile, Jacob appeared before the candidates and explained why he brought them to the island, concluding that one of them needed to replace him. Jack volunteered and Jacob officially made him the protector of the island. ("What They Died For")
Desmond Uncorks the Heart
Jack, Kate, Sawyer, and Hurley ran into the Man in Black, Desmond, and Ben. After Kate tried to kill him, the Man in Black sarcastically warned her to "save her bullets." The groups joined up and went to the Heart. Before he entered, Desmond told Jack of another life where their plane never crashed, and therefore everything that happens on the island is useless, but Jack disregarded this information. Jack and the Man in Black then lowered Desmond into the Heart, something the Man in Black recalled as similar to entering the hatch, a comparison that annoyed Jack. Desmond approached a large rock cork through a field of electromagnetism and managed to remove the cork. The entire island began violently shaking and breaking apart.
Jack and the Man in Black left the Heart. Jack attacked him and made him bleed, causing both of them to realize that the Man in Black was mortal and could be killed, but he knocked out Jack with a rock and fled. ("The End")
Jack vs. the Man in Black
Jack followed the Man in Black to the cliffside that led to Jacob's cave. They had a thunderstruck fistfight, where the Man in Black stabbed Jack in the side (causing a wound Jack would notice in the other timeline). He then attempted to cut Jack's throat (causing another small wound Jack would recognize) before Kate arrived and shot him, noting that she saved him a bullet. Despite his wound, the Man in Black told Jack it was too late as the Island continued quaking, but Jack kicked him off the cliff to the rocks below, and the essence of evil and darkness was dead. ("The End")
Kate and Sawyer fled to Hydra Island after saying goodbye to Jack, Ben, and Hurley. They found Claire there and convinced her to escape with them on the Ajira plane, which Frank, Richard, and Miles had repaired. Everyone boarded the plane and successfully flew off the island. ("The End")
Jack returned to the Heart and passed onto Hurley his job as island protector. Jack then entered the Heart, rescued Desmond, and plugged the cork back into the hole, saving the island from destruction. Jack was engulfed by the light and deposited outside in a river. Hurley asked Ben if he would like to be his assistant and Ben accepted.
Jack was weakened from his exposure to the Light and the wound in his side where the Man in Black had stabbed him. As he stumbled through the jungle, he eventually found the bamboo grove in which he awoke on the day of the crash. Lying down on the ground, Jack was joined for his dying moments by Vincent. As he saw the Ajira plane carrying his friends and the woman he loved fly overhead, he slowly closed his eyes and died.