Albus Dumbledore paced around his office, thoughts swirling around both the vastness of his mind and the infinite depth of his Pensieve. When he defeated the dark wizard Grindelwald, he thought there was no way the wizarding world would ever see another wizard so dark.
He had been so wrong.
Dumbledore had always been thought of as one of the most intelligent wizards of his age; almost prescient, in fact. What nobody knew was how he acquired that knowledge. Most people also did not appreciate how healthy his disregard for the rules was. One of the greatest kept secrets of all time was how much of an opportunist Albus Dumbledore really was. The only difference between him and Grindelwald was that his opportunism was used in the service of others.
Dumbledore understood the power of knowledge. And, while he was aware of the dangers of what he did, he had secretly acquired a Time Turner in his youth and used it regularly to see the past and future. Time travel is dangerous, though, because any change in the past can profoundly change the present.
Imagine time as a river with billions of distributaries constantly branching off to form new rivers with their own billions of distributaries branching off into their own rivers going backward and forward throughout all of time. If you make a change five minutes in the past, you will not have changed very much and that which you have changed hasn't really had enough time to establish itself yet. The farther back you make a change, though, the more of those distributaries, rivers, and potential futures you change.
While any change in the past changes the future in equally profound ways, changes closer to the present are changing a future that has not yet happened; a future that is still somewhat undecided anyway and subject to its own flex. The unpredictability of humanity makes the future unchartable water anyway. Every decision eliminates potential futures while creating new ones, so it is important to not change past events. That is the reason why Dumbledore merely observed the coming storm, rather than going back and trying to avoid it.
During his youth, he had grown very close with his future rival Gellert Grindelwald. They had actually grown as close as two men could grow. Dumbledore and Grindelwald naively believed that all knowledge was worth having, so they stopped at nothing to acquire it. Unfortunately, their love of knowledge also exceeded their love for each other. As their true natures began to unfold, their paths began to diverge.
One of the curses of intelligence is the understanding that power does not come from everybody knowing the same things, but in knowing that which nobody else does. That understanding can create a certain suspicion (a paranoia in some) that a friend knows something and is not telling. Whether it began with Albus or with Gellert, nobody knows, but at some point, each of these dearest of friends realized they needed to begin keeping secrets from each other: Grindelwald to acquire the power he desired and Dumbledore to protect the world from his friend's ambition.
This altruistic compulsion led to Dumbledore spending a great deal of his youth in the past, present, and future secretly observing events and preparing for the war he knew would come. The war that began tonight with the death of a friend and the birth of a savior. The war that would bring friends together, rip families apart, and ultimately lead to his own death at the hands of one of his closest and most trusted allies.
Dumbledore knew all of this, knew it was necessary, yet he still paced around his office trying to find another way… and knowing that he could not.
One of the benefits of time travel most people are aware of is that you can live thousands of years in an instant. One thing most people do not realize about time travel, though, is that you can use it to be in multiple places at once. It was not unusual for several Albus Dumbledores from the past and future to meet with Albus Dumbledore from the present to talk about things that had happened and how they affected things that would happen. One Dumbledore was brilliant. Put twenty of them into one room to brainstorm and the future could be won in an evening.
It was also not unusual for a decision made during those meetings to result in one or more of the future Albus Dumbledores to disappear as his future was avoided, but then new ones would appear as the result of the new potential futures that were created. Sometimes this was the result of a decision and sometimes it was simply the result of sharing information. The beauty of the system was that they all knew what the others had shared with the group, so they were well aware of many of the futures that had been avoided. It was a dizzying process, but he felt it was one of the most important services he could provide mankind. It also resulted in Dumbledore knowing more than any single man should.
On this most important of nights, there was a host of Albus Dumbledores from across time spread throughout the wizarding world acting as scouts. They would come and go from the Headmaster's office at Hogwarts providing the current Headmaster with the information he needed to implement his plan.
Albus heard a popping sound from the corner of his office and turned around to look into the twinkling eyes of himself. Instead of twinkling with intelligence, though, these eyes were twinkling with tears. No words were necessary. Present looked at future, who simply nodded sadly and blinked out of existence. Lily and James Potter were dead, Voldemort was gone, and Harry Potter had just become the fulfillment of a prophecy. Even though the chain of events that had begun this night would not conclude for 17 more years, there was no turning back.
The First Wizarding War was over and the Second Wizarding War had begun.