Why did Voldemort think that luring Harry into the Ministry would work?

Question

Voldemort's plan to get Harry into the Ministry in OOtP didn't make sense. Voldemort was giving Harry the dreams about the Department of Mysteries for months, hoping Harry would get curious and go there. That in itself doesn't make sense. Did he think Harry would just go off to the Ministry after Charms or something? How did Voldemort think that Harry would be able to go?

"Dumbledore never told you?" Malfoy repeated. "Well, this explains why you didn't come earlier, Potter, the Dark Lord wondered why-"

_{Order of the Pheonix - page 786 - Bloomsbury - chapter 35, Beyond the Veil}

Then, when he gives Harry the fake vision about Sirius being tortured, how did he think that would work? When Harry gets it, he immediately tries to tell McGonagall, but she's in St Mungo's.

He waited for the second when the old man's heels disappeared over the treshold into the Great Hall, then ran up the marble staircase and then more staircases toward the hospital wing, hurtling along the corriders so fast that the portraits he passed muttered reproaches, and burst through the double doors like a hurricane, causing Madam Pomfrey, who had been spooning some bright blue liquid into Montague's open mouth, to shriek in alarm.
"Potter, what do you you think you're doing?"
"I need to see Professor McGonagall," gasped Harry, the breath tearing his lungs. "Now...It's urgent..."
"She's not here, Potter," said Madam Pomfrey sadly. "She was transferred to St. Mungo's this morning. Four Stunning Spells straight to the chest at her age? It's a wonder they didn't kill her."
"She's...gone?" said Harry, stunned.

_{Order of the Pheonix - page 730 - Bloomsbury - chapter 32, Out of the Fire}

The only reason he doesn't tell any Order member is because he doesn't think there are any left to find. Umbridge is the reason why Dumbledore and McGonagall are gone, so it was just a fluke that Harry didn't get to tell them. If they had been there, Harry would have told them and they either would have immediately found out that Sirius was okay, or they would have gone to the Ministry themselves and left Harry at Hogwarts.

Plus, he only was able to even get there because he, Hermione and Umbridge went into the forest. All of the factors that led Harry to the Ministry alone were flukes. How could Voldemort possibly think he could get Harry there alone under any circumstances? There was a 99% chance no one would show up because they'd find out Sirius was safe, or that the Order would have shown up without Harry, which would have been no use to him.

Answer 1

As far as I know, there is no canonical answer to this question. The books show us very little of Voldemort directly, but they do show enough to make some educated guesses.

First, Voldemort, as with many people who have managed to achieve anything extraordinary (good or bad), is firmly convinced that he is brilliant. Smart and sensible, however, are not the same thing, and so Voldemort never realizes that some of his plans just aren't practical.

Second, Voldemort is counting on Harry Potter to make the plan work. He offers up as bait something that Harry will be unable to resist acting on. He's seen that Harry is smart, capable and determined. Such people often make things work out by sheer determination, by working around problems as they come up, and making the best of whatever sort of help his friends and random chance provide.

Third, Voldemort truly believes that he needs Harry to get to the Ministry and retrieve the prophecy. Since only he and Harry would be able to retrieve it, it would be much better if Harry did it and then his Death Eaters took it from Harry. Voldemort believes that definitive knowledge of the contents of the prophecy will help him to defeat Harry and succeed in his plan to control the entire world.

Answer 2

Firstly, the Dark Lord already knows how Harry and his friends are resourceful: they have gained access to the hiding place of the Philosopher's Stone as small children. He might also know some parts of the events described in the second and third books. Harry is also older now, so it's reasonable for him to expect that Harry would make some plan this time as well. Going “off to the Ministry after Charms or something” is not really difficult. It's against school rules, but that would not stop Harry.

Secondly, the Dark Lord was careful: he did not go to the Ministry in person. Even if Harry did tell the Order about his visions, and they figured out the Dark Lord was trying to lure him there with a false vision, the Order would go there and attack the Death Eaters, but the Dark Lord himself would still be in safety and there'd be still no convincing the Ministry about his return. He would just figure out some other way to get the Prophecy or Harry later, or have had backup plans already.

Thirdly, disabling Dumbledore and McGonagall wasn't quite a fluke. The Dark Lord was always very successful in turning his enemies against each other, in particular the Ministry not believing Dumbledore made the situation much easier for the Dark Lord during the whole book. The Dark Lord was trying to be careful not to break this situation by not appearing in the Ministry personally, for there'd be a risk he would be seen. He only went when Harry Potter was already there so he was reasonably sure his plan has succeeded. As for Professor Umbridge, Harry would not have needed to get her off his tail if his friends did not insinst on checking up on Sirius in the Headmistress's office.

Answer 3

The principle reason why Voldemort expected Harry to come to the Ministry was that Harry had a penchant for heroics. As Lucius Malfoy told Bellatrix during the confrontation:

"Oh, you don't know Potter as I do, Bellatrix,” said Malfoy softly. "He has a great weakness for heroics; the Dark Lord understands this about him.

Hermione also realized this:

“Okay,” she said, looking frightened yet determined, “I've just got to say this...”

"What?”

“You... This isn't a criticism, Harry! But you do... sort of... I mean — don't you think you've got a bit of a — a — saving-people-thing?” she said.

He glared at her. “And what’s that supposed to mean, a ‘saving-people-thing’?”

“Well... you...” She looked more apprehensive than ever. “I mean... last year, for instance... in the lake... during the Tournament... you shouldn’t have... I mean, you didn’t need to save that little Delacour girl... You got a bit... carried away...”

A wave of hot, prickly anger swept Harry’s body — how could she remind him of that blunder now?

“... I mean, it was really great of you and everything,” said Hermione quickly, looking positively petrified at the look on Harry’s face. “Everyone thought it was a wonderful thing to do —”

“That’s funny,” said Harry in a trembling voice, “because I definitely remember Ron saying I’d wasted time acting the hero... Is that what you think this is? You reckon I want to act the hero again?”

“No, no, no!” said Hermione, looking aghast. “That’s not what I mean at all!”

“Well, spit out what you’ve got to say, because we’re wasting time here!” Harry shouted.

“I’m trying to say — Voldemort knows you, Harry! He took Ginny down into the Chamber of Secrets to lure you there, it’s the kind of thing he does, he knows you’re the — the sort of person who’d go to Sirius’s aid! What if he’s just trying to get you into the Department of Myst — ?”

He knew based on information from Kreacher that kidnapping Sirius would be the thing that would particularly trigger Harry's heroism:

“Oh yes,” said Dumbledore. “You see, Kreacher was not able to betray us totally. He is not Secret-Keeper for the Order, he could not give the Malfoys our whereabouts or tell them any of the Order’s confidential plans that he had been forbidden to reveal. He was bound by the enchantments of his kind, which is to say that he could not disobey a direct order from his master, Sirius. But he gave Narcissa information of the sort that is very valuable to Voldemort, yet must have seemed much too trivial for Sirius to think of banning him from repeating it.”

“Like what?” said Harry.

“Like the fact that the person Sirius cared most about in the world was you,” said Dumbledore quietly. “Like the fact that you were coming to regard Sirius as a mixture of father and brother. Voldemort knew already, of course, that Sirius was in the Order, that you knew where he was — but Kreacher’s information made him realize that the one person whom you would go to any lengths to rescue was Sirius Black.”

It is true that had Order members been around, Harry would probably not have run off to the Ministry on his own, but Voldemort did not give Harry the vision when other Order members were around. He knew that Dumbledore was gone from Hogwarts, and he presumably knew that McGonagall was gone as well. And he specifically made sure no one from the Order was around to receive Harry's communication to Grimmauld Place, as Dumbledore told Harry afterwards:

“The Malfoys — undoubtedly on Voldemort’s instructions — had told him he must find a way of keeping Sirius out of the way once you had seen the vision of Sirius being tortured. Then, if you decided to check whether Sirius was at home or not, Kreacher would be able to pretend he was not. Kreacher injured Buckbeak the hippogriff yesterday, and at the moment when you made your appearance in the fire, Sirius was upstairs trying to tend to him.”

Additionally, Voldemort may have been relying on Harry's past escapades. Notably, Harry went after the Philosopher's Stone and the Chamber of Secrets without obtaining help from any teachers (Lockhart doesn't count), so Voldemort may have expected him to do the same in this instance as well.

As for how Voldemort expected Harry too get to the Ministry, he probably just assumed that Harry would fly there by broom.

All in all, this may have been a somewhat decent plan (especially compared to other Voldemortian plans), and it did in fact work in the end.