Draco replayed the decision in his head while the House Elves appeared, setting up a small table for lunch. After a moment Draco opened his eyes and saw Hermione sitting in her chair, hunched over. After Harry had announced that they'd killed innocent civilians the room descended into several arguments, but Hermione just sat there.

Draco paused. Father's voice quickly listed all of the reasons not to talk to Hermione. Draco ignored him, walked over and sat down beside her, angled slightly away. He didn't want to appear confrontational.

"It was my fault, Hermione." Draco's voice wasn't soothing, didn't sound comforting. Just stating a fact. "I was in command. I gave the order."

"I should have known. I knew something was wrong." Draco saw that Harry walking over towards them. "I knew I'd seen that map before. It was on the news." Her voice was quivering.

"We all knew something was wrong. I felt it. And it came down to a choice. In many ways it's just the counter to my speech after the Purple/Green War. These things happen, and just like we shouldn't beat ourselves up if we die in war..."

"I have less tolerance for murder than death," she glared at him angrily. "And what happened to 'save them all', Draco?'"

"That's the standard we aspire to," Draco said softly. "But it's an impossible standard."

Harry was walking over towards them, and Professor Lockhart had come down from the spectator area. They were both converging on Draco and Hermione.

Draco ignored that, and continued talking "We didn't know what the target was. Shooting it down was a risk. Not shooting it down was a different risk. There were no certainties. I knew that. The cost of being wrong differed, mainly. I couldn't save everyone and that eats at me, a little. But it's only because I messed up. Better to do it now, during a simulation."

Hermione wasn't quite crying, but her eyes weren't dry. Draco felt a spark of rage at Harry. He should have known she'd take this badly. Most of the team wasn't taking it well – losing had that effect on teams – but Hermione felt it deeply. "If I'd killed that many people in real life, I think I'd be hurt. But I can't really feel it. The fact that it upsets you shows you are a good person. Better than me." Draco put his arm around Hermione's shoulder and squeezed, once.

"We should have been able to save everyone," Hermione said.

"It wasn't a fair test," Professor Lockhart said, staring over the heads of Hermione and Draco. "For a number of reasons, but I would say mainly that in terrible situations terrible things happen. And I cannot find it in me to blame people for making the decision that keeps them out of danger. Even if the danger was imaginary."

Harry looked uncomfortable, and Draco glanced around the room. Blaise and Gregory were still yelling, and Neville and Daphne were standing in front of the big map, talking with the Professors. The map showed the positions of the brooms during the attack, right as the pair had broken off from the Doppelganger Defense. Neville pointed out a gap that could have bought them some time, another few seconds.

Gregory had stormed away from Blaise and walked over to Daphne, and said "How could you not contact them?" finger pointing accusingly at her chest. If she'd been a boy, Gregory would have poked her. Draco felt sure that Gregory didn't really feel guilty, but in his mind Gregory had lost and he was lashing out.

"Nobody knows, Gregory." Harry said firmly. "Nobody knows. This was real. It really happened, pretty much like the simulation said." Everyone had stopped arguing and turned to Harry.

"Obviously it's more complicated and didn't involve magic. But the sequence of events is pretty close. You have a crew dealing with harassing brooms, and brooms that color switch, and then a few details get confused and events spiral out of control. I wanted to witness how it happened, so I could learn from their mistake. Maybe I shouldn't have done this. I didn't realize it would hit so hard. I'm sorry."

"There was no right answer," Draco said. "I was just telling Hermione that. If I'd been wrong the other way, we would have all died."

Neville spoke up, "What happened? I mean, something weird happened."

"4474 was renumbered to 4131 automatically by the simulation. I changed the number on you."

"That's cheating," Neville said, over the other protesting voices, but Harry just talked over them.

"Yes, yes. But that's necessary for the fleet to talk to each other. If you don't renumber, then you'll refer to 4474 and the other ships will know it as their number. There was an announcement, but it was in the middle of other things. I didn't expect anyone to notice, and you didn't. People think they notice everything, but you each had a task and with the stress you filtered out the routine background announcements to focus on the threats. You say its cheating, but you didn't really understand the system the simulation used. Not that you necessarily should have, but if you had asked it would have been explained."

"That's not fair," said Blaise.

"There have been studies where they ask people to count the number of times a sports team tosses a ball around a circle. Most people get the number right, but don't even notice the person in the background dressed as a gorilla that comes out and does a little dance," said Harry.

"When did the announcement happen?" Daphne asked.

"During the request for the P-3, shortly after the flight took off. Later on Gregory keyed in the the number which brought up the wrong information, since 4474 now referred to a different plane. Hermione used her wand to pick out the correct plane..."

At the last one Hermione wiped her eyes and a look of despair came across her face. "I wasn't looking for the contact number. Just the altitude. I didn't think to check..."

Harry interrupted her, rather than let her continue to blame herself. "Everyone knew something was wrong, and didn't have time to figure out why. By that point, you had already lost. Draco might have picked the right solution by accident but that would just be luck."

Professor Lockhart nodded and his voice came out low, like a growl. "That's a rather brutal lesson. But correct. You lost when the situation became something beyond your understanding." He paused, then suddenly spoke in a normal voice. "But no harm done! That's why we have these lessons! Very commendable, Mr. Potter."

Harry paused as the last House Elf disappeared. There was a small table set up with assorted meats, cheeses and breads, as well as a bowl of fresh salad and pitchers of tea. A working lunch.

Harry kept talking. "Compared to most students and Muggles you are all battle hardened professionals, but so was the original crew. They argued amongst themselves, possibly even broke military discipline. They knew they were confused, just like you. Already you are so much further ahead than from last year."

"We didn't have time," Daphne's voice was cold, accusing.

"Such is realism," Harry shot back, then grimaced. "I don't want to talk about solutions to this particular problem in general, I just want to hear about what you were thinking as it went on. In wars, lots of people die when you get into situations like this, where people feel threatened and lash out. Muggle Wars kill the innocent much more often than the soldier, but I'm sure it happens in Wizarding Wars as well."

Harry looked around to the Wizarding born. Draco nodded.

"It's not as bad." said Professor Lockhart, "Wizards typically see their victims, but curses still miss and the Killing Curse always finds a victim..."

Draco saw that Hermione had calmed down. She hadn't spoken yet.

"So, I just want to see what we learned. This is a hard problem. You lost the simulation because several improbable factors happened. None of them were rare, but put together it built up a world view that was easy to get wrong. The actual crew got it wrong in the real world, and they had months of training, not a few evenings scattered over six weeks. So, that's the problem. Let's discuss what happened, exactly, for at least half an hour and only then see if we can figure out how to solve it."

Draco sat in the Slytherin common room, facing the roaring fire. They'd talked about the battle for about an hour, everyone eventually grabbing some food. After they'd ended the session Draco had come back and spent the rest of his day sitting. Lodged in front of the fire thinking, pausing only to eat dinner. He'd considered today's lessons and compared them to the past two years. People came and went in the common room, Draco barely noticed. Draco only glanced up at Gregory when he said it was time for dinner.

Draco shook his head and fell back into thought.

Hours later, well, after the sun went down, Draco went to his room, got a small scroll, and sat down at his desk. He sharpened a quill and started to write.

The keys to defeating a clever enemy are several fold:

There must be some information that the enemy does not know, or knows incorrectly.

If there is a solution that the enemy can spot, you can still make it difficult by creating so many possibilities that they cannot be evaluated in time. Suppose there are two key facts that the enemy needs to piece together. Even if they know both, if they have to compare 50 facts it may be impossible to link them. And if the fact has to link through an intermediate fact, it becomes even harder.

Draco considered this. Harry had full advantage of the methods, but Harry's lesson on maths had been revealing. Sometime you had to have a key insight, and how difficult that is depends on how many facts you have to compare to reach it. If you have just two facts, you can draw a line between them in only a single way. If you have three facts you draw three lines, it makes a triangle. If you have four facts, you draw a square, but you can also connect the corners. Six possibilities. Five facts and you have a pentagon with five internal lines. The number of lines grows faster than the number of dots. And that's just if you have to compare two facts. If you have to group three facts together, you matched pairs of lines. Draco's Arithmancy wasn't up to the task of figuring out the exact number, but it didn't matter. If you got to even fifteen or twenty facts, it would become impossible.

Too much information can, paradoxically, make decisions less obvious.

Motives and Goals are key facts, easily hidden in some cases.

Even in a puzzle, an ambush is the best way to go.

Coupled with stress and time pressure, the enemy will discover themselves in a situation where they can only win by luck.

Try to reduce your enemy's knowledge of their current situation.

Ideally, take action to counter luck and make it so that either obvious decision is the wrong one.

Draco glanced down at the paper, read over it once, then crumpled it up. He stared at the ceiling for a few moments, then nodded to himself. Draco walked back to the common room and tossed the crumpled scroll into the fire and sat back down into his chair.

For the first time all day he smiled.

Author's Note - As several people on r/hpmor noticed, this scenario is (loosely) based on the USS Vincennes which shot down Iran Air Flight 655 over the Straits of Hormuz on July 3rd, 1988. The incident is modeled in "Sources of Power: How People Make Decisions" by Gary Klein, which goes into more detail than the Wikipedia account.

I have not independently confirmed the details provided by Klein. There are considerable differences between his review and the information presented on Wikipedia. My presentation is based on one version of the facts, facts that were hotly debated for years.

I have changed some details (with an eye towards simplification, but also to make the situation more stressful). In the real world the Vincennes was responding to a hostile attack on an allied ship at the time, so open hostilities were occurring. The severity of this attack is under dispute. Others (including some in the US Navy) believe the Captain of the Vincennes over-reacted, but for purposes of the simulation, Harry simulated a true threat.

The AEGIS Targeting computer labelled 4474 "presumed hostile" because it had delayed from its scheduled departure time. Further confusing the issue was that the computer re-labeled the target from 4474 to 4131 shortly after take off and in the confusion most of the crew did not hear this announcement. If anybody did hear, it apparently did not register. Which means that some crew typed in the number 4474 on their consoles and reported the airspeed indicators of the new target 4474, an A-6 which was descending and speeding up.

Taking all this (mis)information together, some crew interpreted the commercial airliner as "making an attack run" against the ship. Iran Air Flight 655 did have a civilian transponder code, but the Iranian Military sometimes spoofed civilian transponders on their military planes and had done that recently.

There is still some question as to why the commercial airliner did not respond to clarify. The Vincennes requested contact on civilian channels (3 times, according to Wikipedia), but apparently did not have the communication capacity to contact commercial air traffic control to ask them to relay the message. (According to Klein). Why IA 655 did not respond is a mystery.

Records show that the crew was divided before the captain gave the order to fire.

The entire incident - from takeoff until the USS Vincenness fired - lasted eight minutes.

None of the 290 people onboard IA 655 survived.