Ender's Game has many significant, major events that pinpoint the twists and turns of the story, including many that happen without Ender knowing about them, at least for a while. For example, the government decides to raise themselves a military genius and they take precautions to make sure that the person they choose feels isolated and alone to make sure he isn't hindered by empathy and friendship when it comes to making difficult decisions. While this isn't portrayed as a major event in the book, exactly, it sets everything else into motion.
A selective overview of the major events of the book is as follows:
Ender kills a boy called Stilson, who has been bullying him. It sets the stage for Ender's later guilt, as well as fear that the Battle School is turning him into his cruel brother Peter. The fact that the boy's death is hidden from him also emphasizes the actions of the government trying to create their military commander at all costs, including pardoning murder.
Ender is persuaded to go to the Battle School. He does this out of love, fear, and a sense of duty.
Ender is separated from friends and promoted as soon as he finds success. The higher-ups can't have him building relationships, and they are in a hurry to push him further as soon as he becomes comfortable. They do this to train his mind to constantly adapt.
Ender wins an important battle by disobeying orders. This makes him unpopular with other students and assures the commanders that they've made the right choice with him.
Ender's brand-new army is driven to the edge. The Battle School keeps adding pressure on him, limiting his preparation time.
Ender kills another boy, defending himself. Once again, he is not told. The government needs him desperately, while Ender resents everything happening to him more and more.
While he's at the school, Ender's brother Peter and sister Valentine become major political figures. They are able to use their inherent intelligence to influence the politics of the world without revealing their youth.
Ender gratuates from the Battle School.
The government brings in Valentine to convince Ender to continue his training. While he knows they're manipulating him by letting him see his beloved sister, Ender agrees.
Ender is told that the government has been pushing him so hard because, after two invasions, Earth is finally ready to battle their enemies: the Buggers.
Ender wins the final exam battle and is told he has wiped out the entire Bugger race. He finds out at last that the simulations he thought he was playing were the war itself, and he has saved Earth at the cost of killing off an entire race.
Earth agrees to a worldwide peace.
Ender and Valentine go to one of the occupied Bugger planets, and Ender finds a Bugger queen egg. He realizes that the Buggers were intelligent beings who knew they had no way out and have left him the queen, which he now has a chance to save.
Ender becomes the "Speaker for the Dead" and promises to find the egg a new, safe home.