Konoha Crush

The Invasion of Konoha was the planned invasion of Konohagakure by Otogakure and Sunagakure during the Chūnin Exams.

Background
Sunagakure had for years suffered a series of budget cuts by the daimyō of the Land of Wind, weakening the village and thus rerouting ninja missions to Konoha. Orochimaru, the leader of Otogakure, had years earlier left Konoha after having not been accepted for the position of Hokage. With both villages having vendettas against Konoha, Orochimaru approached the Fourth Kazekage with the proposition of a joint invasion. Orochimaru would at some point assassinate and impersonate the Kazekage to make controlling Sunagakure easier, so it is unclear how much of the plan was his idea.

The attack was meant to be fought on two fronts: Sunagakure's jinchūriki Gaara would make it to the final rounds of the Chūnin Exams taking place in Konoha and would release the One-Tailed Shukaku from the center of the village. While Konoha was preoccupied with Shukaku, Sunagakure and Otogakure forces would break through the walls surrounding the village. Orochimaru, having only one goal for the invasion, would fight and kill his old teacher and the current Hokage, Hiruzen Sarutobi. Though Konoha never knew the exact details of the invasion or who would be invading, it suspected an attack and had its forces prepare for the worst.

Invasion
When Gaara was injured by Sasuke Uchiha during the finals, he was left unsuited for fulfilling his role in the invasion. Orochimaru's subordinate, Kabuto Yakushi, instead cast the Temple of Nirvana Technique on the stadium where the finals were being held, putting most present to sleep and signaling the invasion. The Konoha ninja that were present and able to fend off the genjutsu immediately engaged the Sound ninja that had earlier infiltrated the stadium. Gaara and his siblings fled the village, and Orochimaru, removing his disguise as the Kazekage, began fighting Hiruzen.

On Konoha's outskirts, Sand and Sound ninja, with help of some of Orochimaru's snakes summons, punched holes in the village's defences, eliminated the border guards, and moved in to the village. All combat ready-ninja sought out the invaders to give the citizens time to escape to the safe area behind the Hokage Monument. Having taken the villagers by surprise the Sand and Sound were able to claim some early victories, in part benefited by the snakes rampaging throughout the village.

As time went on and Konoha's more seasoned ninja came to the village's defence, the tide began to turn against the invaders. Jiraiya stepped in to stop Orochimaru's snakes and most of the Sound ninja at the stadium had been eliminated. Orochimaru, though having managed to bring about Hiruzen's death, had been badly injured by the loss of his arms. Unable to fulfil whatever plans he had had for afterwards, he fled with his bodyguards, the Sound Four. Gaara was defeated by Konoha's own Jinchūriki, Naruto Uzumaki, and fled at the climax of the battle. The remaining Sound ninja followed suit and the Sand, knowing the battle was lost, retreated.

Aftermath
Having realized that something had happened to their Kazekage, Sunagakure scoured the Land of Wind looking for him. They found his body and, realizing they had been manipulated into the invasion by Orochimaru, immediately informed Konoha of what had happened. They surrendered to Konoha and offered their apologies, which Konoha accepted. The two villages formed a peace treaty and Sunagakure, inclined to make amends, would thereafter come to Konoha's aid if ever there was need. Sunagakure, despite the casualties they had received in the botched invasion, would benefit greatly from Konoha's influence, adopting the Leaf's structure for a Ninja Academy and in later years receiving help when their Fifth Kazekage, Gaara, was kidnapped.

Konoha, though victorious, had suffered heavy losses in the village's defence, chief among them being the loss of the Hokage. Fearing that other parties might prey on them in their weakened state, Konoha earnestly tried to find a new Hokage. At first, the councilors decided that Jiraiya would take the fifth Hokage's place. Jiraiya, however, refused the job, claiming that he was too "irresponsible". Later, Tsunade would eventually be persuaded to replace Hiruzen, and she oversaw the village's reconstruction effort. Until Konoha's forces could otherwise be replenished, the surviving ninja were forced to go on more missions than usual and of higher ranks than they were accustomed. For a while the village would be understaffed, to such an extent that when Sasuke Uchiha defected to Orochimaru a team of Genin (and some backup from Sunagakure) was the best that could be sent to stop him. Konoha had been restored to its original operating capacity by Part II, and was presumably better prepared when the leader of Akatsuki, Pain, invaded the village.

Trivia

 * During the Invasion alone, there was a total of 8 summonings. 2 were made by Orochimaru (The First Hokage, and the Second Hokage), 2 by Naruto (Gamakichi and Gamabunta), 1 by Jiraiya (Gamahiro), 1 by Kakashi (Pakkun), 1 by Sand Ninja, a snake, and 1 by Hiruzen (Monkey King: Enma)
 * From what we know about the participants of the battle, there were at least 100 Sand Ninja crossing the walls, over a dozen Sound Ninja in the stadium, an unknown number in Konoha itself, and besides the elites, we don't know the defenders in the village.