I believe the correct answer is the most straightforward one: Dumbledore did not know everything about everything until just before we did. So let us explore what Dumbledore knew and when—a recurring question to keep in mind in order to maintain a sense of perspective.
1992 and Earlier
Up until that very significant conversation in Chamber of Secrets, Dumbledore did not even know whether Voldemort had created Horcruxes or not. It may seem obvious in retrospect that he had. But consider this very telling line from Voldemort’s monologue in Goblet of Fire: “. . . it appeared that one or more of my experiments had worked . . . for I had not been killed.” (GF653) That means that Voldemort had made several “experiments” in his quest for immortality. How was Dumbledore to know what Dark magic Voldemort had performed? All Dumbledore knows is that Voldemort spent about twenty years exploring ways to become immortal, and Merlin knows which one he actually chose.
But suppose Dumbledore realized that Voldemort might have made a Horcrux. Dumbledore would not know that Voldemort made more than one because doing so is completely unprecedented. He tells Harry, “[Voldemort] was referring to his Horcruxes, Horcruxes in the plural, Harry, which I do not believe any other wizard has ever had.” (HBP501-502) Just think of how horrified Slughorn is at Tom Riddle’s suggestion of multiple Horcruxes in that crucial recovered memory (HBP498). If Dumbledore was contemplating different methods of immortality, I don’t think he would consider multiple Horcruxes.
Let’s say for argument’s sake he suspects that Voldemort made one Horcrux. The only one he reasonably could have found, with the information available to him, is the locket or the ring. If Dumbledore suspected that the cup or the diary was the Horcrux, he would be at a complete dead end, since he can’t exactly go snooping through the possessions of all the former Death Eaters.
So Dumbledore might suspect that there’s one Horcrux out there. Having been collecting memories about Voldemort already, there are several possible candidates, but Dumbledore has no way of knowing which one is the Horcrux or where it will be found. Dumbledore is not even sure whether Voldemort has a Horcrux or if he used some other Dark magic. But with Voldemort temporarily out of the picture, Dumbledore can focus on other endeavors: rebuilding the world after the first Voldemort War, enacting his scheme surrounding the Sorcerer’s Stone, and dealing with the Chamber of Secrets.
End of Chamber of Secrets
The first breakthrough comes at the end of Chamber of Secrets, when Harry comes into Dumbledore’s office, triumphantly wielding a sword and a destroyed diary, telling strange tales of meeting a teenaged Voldemort. Dumbledore explains his thought process to Harry during their Horcruxes 101 lesson.
“I was almost sure of it. The diary had been a Horcrux. But this raised as many questions as it answered.
“What intrigued and alarmed me most was that that diary had been intended as a weapon as much as a safeguard.”
[. . .]
“The careless way in which Voldemort regarded this Horcrux seemed most ominous to me. It suggested that he must have made—or been planning to make—more Horcruxes, so that the loss of his first would not be so detrimental.” (HBP500-501)
Dumbledore gets confirmation that Voldemort did indeed make a Horcrux, but alarm bells go off in his head. If Voldemort treated his Horcrux as expendable, that means that Voldemort must have made more than one. This is a mind-blowing idea, but it appears to make sense. So now Dumbledore starts thinking, Just how bad is the situation? Did Voldemort make two Horcruxes (unprecedented!), or did he go even further than that and make three? The answer to “not who, but how?” is even more important than anyone initially thought.
While this is all quite frightening to contemplate, Dumbledore is not particularly frantic just yet. Voldemort is still somewhere in Albania, alone and almost powerless. The Sorcerer’s Stone is destroyed, one Horcrux—Tom Riddle’s Diary—is down, and things are looking rosy for the moment. I’m sure Dumbledore starts poring over his memories of Tom Riddle, doing his research, trying to identify what could possibly be a Horcrux.
However, this is all soon driven from his mind, and Horcruxes are put on the backburner for two years. Because if Dumbledore was hoping for some down time after the whole Chamber of Secrets debacle, he’s about to get some really unwelcome news about a prisoner in Azkaban.
Chapter 3:
Albus Dumbledore and the
Prisoner of Azkaban
Prisoner of Azkaban is a unique book among the seven for many reasons, chief among them the absence of Voldemort. It is also unique because it’s the only book where the reader is pretty much on the same page as Dumbledore for most of it—believing in Sirius Black’s guilt and at a loss as to how Sirius is achieving things.
Prisoner is also where Dumbledore’s choice of D.A.D.A. professors begins to inform the story. Some context: Voldemort cursed the position when he applied for it and was rejected, which can be placed with reasonable certainty in the winter of the 1956-1957 schoolyear. (See Appendix A.) Let us take Dumbledore’s statement at face value, that they “have never been able to keep a Defense Against the Dark Arts teacher for longer than a year since [Dumbledore] refused the post to Lord Voldemort.” (HBP446)18 That would mean Lockhart had been the thirty-fifth D.A.D.A. teacher in as many years.
As Lockhart’s appointment indicates, Dumbledore has utterly exhausted the pool of potential job applicants who would actually be appointed purely for their pedagogical prowess. His appointments henceforth will be informed by ulterior motives. With Lupin, it’s two-fold. On the one hand, Dumbledore wants to help Lupin after Umbridge’s anti-werewolf legislation makes it impossible for him to find work (OP302). On the other hand,
