Strategy and tactics of Dumbledore.

Dumbledore is the headmaster of Hogwarts School of Witchcraft and Wizardry, and at the same time the main antagonist of Voldemort, who aims to control not just the Wizarding world, but the Muggle (non-magical) world as well.
Both must develop a strategy to succeed in their purpose.
In this document I indicate the possibilities available to Dumbledore, and how well in my opinion he uses them.
These possibilities he has are based on what other persons in the Harry Potter series can do.
So he could and should have produced a Marauder's Map, used the Priori Incantato spell, used Legimency and produced a taboo.
Maybe Specialis Revelio could have been useful as well. It is not clear what it exactly can do.

The Marauder's Map.
It produces a detailed layout of Hogwarts, including its occupants, secret passageways (and instructions on how to access them), and other mysteries, though several locations (such as the Room of Requirement and the Chamber of Secrets) do not appear on the map, either because the Marauders did not have any knowledge of them, or—in the case of the former—they are Unplottable. Furthermore, Animagus disguises, Polyjuice Potion or Invisibility Cloaks cannot fool the map.

Priori Incantato.
This spell reproduces an image of the last spells the wand is used for.

Legilimens.
Allows the caster to delve into the mind of the victim, allowing the caster to see the memories, thoughts, and emotions of the victim. Dumbledore was a good Legilimens (he says so himself in book V).

Taboo.
Voldemort used some form of a taboo spell to discover persons that used his name.

Book I: Harry Potter and the Philosopher's Stone (Sorcerer's Stone).
Harry Potter becomes a pupil of Hogwarts. Voldemort tries to find the Philosopher's Stone (Sorcerer's Stone) to regain his normal life. He does this by using Quirrell. Both Quirrell and Harry and his friends pass the obstacles and reach the Mirror of Erised. Dumbledore had foreseen that the Mirror would show Voldemort/Quirrell only themselves making the elixir of life, as they wanted to use the Philosopher's Stone for that; Harry was able to see and get the Stone in the Mirror because he wanted to find it but not to use it. Therefore Harry gets the Stone. Quirrel tries to grab the Stone. Harry is saved by Dumbledore.
This rises a few questions:
- Were the (very ingenious) obstacles the best solution for protecting the Stone? Obviously not: apart from the last one Quirrel and Harry and friends solved them both. And Harry got the Stone from the Mirror of Erised, and Quirrel/Voldemort nearly got it.
Could Cumbledore not have invented something better? E.g. using the Fidelius charm?
- No doubt Dumbledore could have discovered the possession of Quirrel by Voldemort using Legilimency.
- Priori Incantato and Legilimency could have been used to find out who tried to kill Harry during his first Quidditch match.
 
Book II Harry Potter and the Chamber of Secrets.
The book describes Harry's second year at Hogwarts School of Witchcraft and Wizardry, during which a series of messages on the walls on the school's corridors warn that the "Chamber of Secrets" has been opened and that the "heir of Slytherin" will kill all pupils who do not come from all-magical families. These threats are followed by attacks which leave residents of the school "petrified" (that is, frozen). Throughout the year, Harry and his friends Ron Weasley and Hermione Granger investigate the attacks, and Harry is confronted by Lord Voldemort, who is attempting to regain full power.
Harry again has a very narrow escape.
It appeared Ginny was possessed by Voldemort/Tom Riddle.
Dumbledore might have discovered that Ginny was possessed by using Legilimency.

Book IV Harry Potter and the Goblet of Fire.
In the fourth book Harry's name is put in the Goblet of Fire, so he had to compete in the Triwizard Tournament. Some dark wizard must have put his name in it.<br>
Priori Incantato could have been used to find out who put Harry's name in the Goblet of Fire.
Dumbledore should have used his Marauder's Map to discover that Alastor "Mad-Eye" Moody was not the person he seemed to be. Polyjuice Potion cannot fool the map.
The same applies to Harry Potter. At one moment he discovered this, but did not draw the correct conclusion.
If he had looked at any other moment he would have discovered the truth about Mad-Eye.
Taboos on the names of Death Eaters would also have been very informative. Book V Harry Potter and the Order of the Phoenix.
Harry and his cousin, Dudley, are attacked by dementors. Dolores Umbridge has sent them.
This could have been discovered by Legilimency.BR> Taboos on the names of (escaped) Death Eaters would have been very useful. Book VI Harry Potter and the Half-Blood Prince.
In this book Voldemort has ordered Draco Malfoy to kill Dumbledore. Dumbledore understood this, but decided not to act on this information.
Malfoy repaired the Vanishing Cabinet and its connection to the one with Borgin and Burkes, using it to smuggle Death Eaters into Hogwarts. Dumbledore could have discovered this using Legilimency.
He also could have prevented the accidental attacks on Katy and Ron, which could have been fatal.
He should have given more information to Harry Potter.