Madara Uchiha, according to the Fifth Mizukage, "know everything." That said, he usually uses his knowledge to coerce or manipulate others in doing his bidding for him; also, his goals are carefully laid out with reasons as to why he is trying to accomplish them. As we know, the Nine-Tailed Fox's attack on Konohakagure has been one of the most important events in the Naruto series, but it seems as if the more we learn about it, the more mysterious the event itself actually becomes.
Madara has connections with the entire ninja world, once becoming Mizukage of Kirigakure, as he does subordinates. He has witnessed a great deal in his lifespan, and also accumulated an extraordinary amount of fame in the eyes of ninja, becoming one of the most widely known figures in history, not without its price though. After supposedly being fatally wounded by Hashirama Senju, the First Hokage, he is revealed to have survived, yet faded into obscurity in the process. The reason for this is understandable, but if Kisame addresses Madara as "Mizukage," then a question emerges: if Madara wanted to hide himself in the shadow of Akatsuki, why would he become the Kage of a village, where he would once more be thrown into the limelight? I suppose that Madara wanted some sway over the events in the ninja world, particularly the situation with the Uchiha clan in Konoha. As Madara tells Sasuke, the Second Hokage gave the Uchiha a special position of trust - the police force, but its true intention was to distance the Uchiha from any direct political positions in the village. The Mizukage could have made some kind of objection to such treatment.
All of this leading up to the point I want to make - before and up to his battle with Hashirama, Madara's life was one long battle, but it seems that things became awfully quiet after his defeat. Konoha stabilized, as did the other villages; but it would be without a doubt the assumption that naturally, Madara kept a close eye over the affairs of the ninja world, and suited his plans accordingly. He became bitter towards both Senju and Uchiha, and in my mind, saw of a perfect way to tarnish Konoha's reputation permanently. That shall be addressed a little later. First, Madara takes interest in both Naruto and Sasuke's development, and likes to "stockpile good eyes," in his own words - he tells the story of the Sage of the Six Paths to the five Kages, to Naruto, Kakashi, Yamato, but not to Sasuke. Why? Perhaps when Sasuke finds out that he is being used (when he thinks he is the one using Madara) for a plan known as the "Moon's Eye Plan," he will disband himself from Akatsuki.
Now, coming to the point I want to make. The Nine-Tailed Fox's attack on Konoha complements Madara's plans perfectly. Upon finding out that Kushina was the previous host of the Nine-Tailed Fox, the storyline suddenly took another dramatic twist. But another question: Madara, while telling Sasuke the truth about Itachi, mentions Konoha's beginnings and the fact that Hashirama divided the power of the tailed beasts so each would receive an equal amount; but who received the Nine-Tailed Fox? Kushina is from the Land of Whirlpool - this dispels the notion that Konoha kept the Nine-Tails, so why was it handed over to the Land of Whirlpool? Naturally, Hashirama would make the argument that it was most fitting for Konoha to keep the Nine-Tails, because there would be no danger in doing so. He did kill Madara, did he not? Would perhaps the most powerful ninja in history make such an enormous blunder by leaving Madara alive? Or was Hashirama just making sure, and handed over the Nine-Tails to a country that was not well-known? Wherever the Nine-Tailed Fox went, Madara found out about its whereabouts, and used it to his advantage.
Earlier, we mentioned an ideal way to ruin Konoha's reputation: well, here it is. Being that Kushina was the previous, and one of the perhaps many hosts of the Nine-Tailed Fox, my theory is that Madara captured her (something that might have been attempted before with Kumogakure's attempted capture of her as a fifteen year old, or is this just coincidence?), with the birth of Naruto occurring somewhere during this time, and forcibly opened the seal on the Nine-Tails ("Uchiha's powers are the only thing that can control the Kyubi"), in other words making her lose control of both herself and the beast sealed within her. Minato, while meeting with Naruto for a brief period, described Madara as "a very powerful ninja" who "saw through everything" he did. It is a fact within the Naruto world that when a host loses control of the beast sealed within them, the tailed beast takes them over and kills them in the process. We do not know whether Minato knew Madara specifically was behind the attack, or whether he realized that when speaking with Naruto, but to me, Madara completely expected Minato's response. He intended for the Nine-Tailed Fox to be sealed within Naruto, which does two things: first, it removes potential obstacles from his plans (both Kushina and Minato) and also sends the village into upheaval. The Nine-Tailed Fox must be sealed last in the statue (a fox that is free to roam around is no good to Madara), so in essence Madara removed it from its current host, freed it, then sealed it into another: Naruto Uzumaki. It makes sense for Madara to have drawn out the Nine-Tails from Kushina and forcing her to transform into the beast, because Kushina said to Naruto her chakra was included in the sealing process - Minato sealed the Nine-Tailed Fox (in a sense, Kushina) into Naruto, which explains why she is part of the seal.
Getting on to the second point: the village's drastically weakened situation. Characters have pointed out that after the Nine-Tails' rampage came to an end, Konoha was left vulnerable and might be attacked at any moment. Disaster was avoided on the outside, but internally Konoha was threatening to crumble. Many ask why Madara did not simply continue his attack while Konoha could have been finished, but the answer is he did not need to. The elders had already suspected an Uchiha was behind the incident (in a way they were right, but not the Uchiha they expected), which incited anger within the clan, who planned to overthrow the government that was currently in power. Madara, as mentioned earlier, was bitter towards both Senju and Uchiha, and using the Nine-Tailed Fox, got exactly what he wanted. Killed two powerful people in a very short amount of time, then slaughtered many more soon after. The Uchiha clan massacre, while having been executed on Itachi's orders from Konoha, might have been important for more reasons than one. Sasuke was the only who survived the killings, and it is likely that Madara saw something (despite his being a seven year old child), a potential that far surpassed Itachi's (which, interestingly enough, is what Orochimaru said during the Chunin Exams when placing the Cursed Seal Mark on Sasuke). Constantly watching Sasuke and Naruto climb in the ranks of the ninja world, it makes sense why he wants the two to clash for one final time (and if they do not, he will make them, which can be accomplished given his coercive nature). "You two have been chosen by fate," says he, and it does not take extensive thought to realize why. The survivor of the massacre of the one most powerful clans in ninja history, and a possible descendant of the founder of the ninja world, the Sage of the Six Paths. All, if not the entire series, has been going along with Madara's true plan - it is only fitting that these two meet by a hand higher than chance.
Madara has connections with the entire ninja world, once becoming Mizukage of Kirigakure, as he does subordinates. He has witnessed a great deal in his lifespan, and also accumulated an extraordinary amount of fame in the eyes of ninja, becoming one of the most widely known figures in history, not without its price though. After supposedly being fatally wounded by Hashirama Senju, the First Hokage, he is revealed to have survived, yet faded into obscurity in the process. The reason for this is understandable, but if Kisame addresses Madara as "Mizukage," then a question emerges: if Madara wanted to hide himself in the shadow of Akatsuki, why would he become the Kage of a village, where he would once more be thrown into the limelight? I suppose that Madara wanted some sway over the events in the ninja world, particularly the situation with the Uchiha clan in Konoha. As Madara tells Sasuke, the Second Hokage gave the Uchiha a special position of trust - the police force, but its true intention was to distance the Uchiha from any direct political positions in the village. The Mizukage could have made some kind of objection to such treatment.
All of this leading up to the point I want to make - before and up to his battle with Hashirama, Madara's life was one long battle, but it seems that things became awfully quiet after his defeat. Konoha stabilized, as did the other villages; but it would be without a doubt the assumption that naturally, Madara kept a close eye over the affairs of the ninja world, and suited his plans accordingly. He became bitter towards both Senju and Uchiha, and in my mind, saw of a perfect way to tarnish Konoha's reputation permanently. That shall be addressed a little later. First, Madara takes interest in both Naruto and Sasuke's development, and likes to "stockpile good eyes," in his own words - he tells the story of the Sage of the Six Paths to the five Kages, to Naruto, Kakashi, Yamato, but not to Sasuke. Why? Perhaps when Sasuke finds out that he is being used (when he thinks he is the one using Madara) for a plan known as the "Moon's Eye Plan," he will disband himself from Akatsuki.
Now, coming to the point I want to make. The Nine-Tailed Fox's attack on Konoha complements Madara's plans perfectly. Upon finding out that Kushina was the previous host of the Nine-Tailed Fox, the storyline suddenly took another dramatic twist. But another question: Madara, while telling Sasuke the truth about Itachi, mentions Konoha's beginnings and the fact that Hashirama divided the power of the tailed beasts so each would receive an equal amount; but who received the Nine-Tailed Fox? Kushina is from the Land of Whirlpool - this dispels the notion that Konoha kept the Nine-Tails, so why was it handed over to the Land of Whirlpool? Naturally, Hashirama would make the argument that it was most fitting for Konoha to keep the Nine-Tails, because there would be no danger in doing so. He did kill Madara, did he not? Would perhaps the most powerful ninja in history make such an enormous blunder by leaving Madara alive? Or was Hashirama just making sure, and handed over the Nine-Tails to a country that was not well-known? Wherever the Nine-Tailed Fox went, Madara found out about its whereabouts, and used it to his advantage.
Earlier, we mentioned an ideal way to ruin Konoha's reputation: well, here it is. Being that Kushina was the previous, and one of the perhaps many hosts of the Nine-Tailed Fox, my theory is that Madara captured her (something that might have been attempted before with Kumogakure's attempted capture of her as a fifteen year old, or is this just coincidence?), with the birth of Naruto occurring somewhere during this time, and forcibly opened the seal on the Nine-Tails ("Uchiha's powers are the only thing that can control the Kyubi"), in other words making her lose control of both herself and the beast sealed within her. Minato, while meeting with Naruto for a brief period, described Madara as "a very powerful ninja" who "saw through everything" he did. It is a fact within the Naruto world that when a host loses control of the beast sealed within them, the tailed beast takes them over and kills them in the process. We do not know whether Minato knew Madara specifically was behind the attack, or whether he realized that when speaking with Naruto, but to me, Madara completely expected Minato's response. He intended for the Nine-Tailed Fox to be sealed within Naruto, which does two things: first, it removes potential obstacles from his plans (both Kushina and Minato) and also sends the village into upheaval. The Nine-Tailed Fox must be sealed last in the statue (a fox that is free to roam around is no good to Madara), so in essence Madara removed it from its current host, freed it, then sealed it into another: Naruto Uzumaki. It makes sense for Madara to have drawn out the Nine-Tails from Kushina and forcing her to transform into the beast, because Kushina said to Naruto her chakra was included in the sealing process - Minato sealed the Nine-Tailed Fox (in a sense, Kushina) into Naruto, which explains why she is part of the seal.
Getting on to the second point: the village's drastically weakened situation. Characters have pointed out that after the Nine-Tails' rampage came to an end, Konoha was left vulnerable and might be attacked at any moment. Disaster was avoided on the outside, but internally Konoha was threatening to crumble. Many ask why Madara did not simply continue his attack while Konoha could have been finished, but the answer is he did not need to. The elders had already suspected an Uchiha was behind the incident (in a way they were right, but not the Uchiha they expected), which incited anger within the clan, who planned to overthrow the government that was currently in power. Madara, as mentioned earlier, was bitter towards both Senju and Uchiha, and using the Nine-Tailed Fox, got exactly what he wanted. Killed two powerful people in a very short amount of time, then slaughtered many more soon after. The Uchiha clan massacre, while having been executed on Itachi's orders from Konoha, might have been important for more reasons than one. Sasuke was the only who survived the killings, and it is likely that Madara saw something (despite his being a seven year old child), a potential that far surpassed Itachi's (which, interestingly enough, is what Orochimaru said during the Chunin Exams when placing the Cursed Seal Mark on Sasuke). Constantly watching Sasuke and Naruto climb in the ranks of the ninja world, it makes sense why he wants the two to clash for one final time (and if they do not, he will make them, which can be accomplished given his coercive nature). "You two have been chosen by fate," says he, and it does not take extensive thought to realize why. The survivor of the massacre of the one most powerful clans in ninja history, and a possible descendant of the founder of the ninja world, the Sage of the Six Paths. All, if not the entire series, has been going along with Madara's true plan - it is only fitting that these two meet by a hand higher than chance.