Despite their reputation, prophecies in Greek mythology are actually fairly nice. You’re allowed to mess with them and control how they come true, and the universe doesn’t destroy you for it unless you try to stop them. The prophecy is just giving true information about the future, which is basically the best way it can work in terms of twisting it to your advantage. And if they had just used it, either side in the Trojan War could have guaranteed victory easily.
They had way too many prophecies floating around. Any time anyone wants to do anything, you can be fairly sure that they’ll be stepping on the toes of destiny somehow. And they really should have taken advantage of it. Imagine Professor Quirrel or any other actually smart character being in the Trojan War and being told about the prophecies. They would have been unbeatable just by sheer force of fate. (Evil Overlord Rule 111: I will offer oracles the choice of working exclusively for me or being executed.)
To start with, the Trojans had a ridiculously obvious way to never lose. They had a prophecy that the city would be safe as long as a particular artifact was inside it. You’d think armed guards would be a no-brainer, as would keeping the location secret. The concept of a “decoy” could also come in handy. (Evil Overlord Rule 27: I will never build only one of anything important.) Who told Odysseus where to find it? The Greek king’s ex-wife. Yep, the Trojans let her (of all people) know that. (Evil Overlord Rule 133: If I find my beautiful consort with access to my fortress has been associating with the hero, I’ll have her executed. It’s regrettable, but new consorts are easier to get than new fortresses and maybe the next one will pay attention at the orientation meeting.)
Do these people even have a concept of information you might not want to tell the whole city? Yes they do, actually. They successfully kept secret the fact that such a prophecy existed for years, leaving the Greeks fighting an unwinnable war. Good for them. (Evil Overlord Rule 198: I will remember that any vulnerabilities I have are to be revealed strictly on a need-to-know basis. I will also remember that no one needs to know.) But then after the Greeks found out, the Trojans didn’t know they knew, so they had no way of taking precautions and the Palladium just got carried right out by an awesome Achaean. (And Odysseus.) Except of course that the Trojans should have protected it anyway, because unbeatability is a property that you probably want to keep. Especially when it’s as simple as “protect the magic item.”
But lest I be accused of being unfair to the Trojans, the Greeks passed up a way cooler guaranteed win. There was a lesser-known prophecy that provides a perfect example of a situation that’s just begging for some rules lawyering. An oracle said that the first Achaean to walk on the enemy’s land would be the first to be killed. This being the ancient Greeks, some guy named Iolaus declares it to be totally worth it and dies gloriously in battle.
He did get to kill four of the enemy, get famous, have Euripides write a play about him, and get shrines built by his cult following. All of which, in the ancient Greek mindset, is more important than being alive. They would look back on that and see a successful prophecy that worked out for everyone; I would look at it and wonder what they were thinking.
The obvious thing to do is have someone volunteer, then get them back on the ship and protect them like the Trojans should have protected the Palladium. The Trojans are then stuck fighting through an army of men they can’t kill. It doesn’t go well for them. And while they’re at it, they should publicize that particular prophecy and let slip the wrong name to the Trojans. (Evil Overlord Rule 221: Whatever my one weakness is, I will fake a completely different one.)
Obviously this can’t work for the Greeks; the writers wouldn’t let it. They’d probably have Iolaus get shot in the back or something, so he’s just as dead as in the original but he doesn’t end up being worshipped. So have Achilles do it instead. He has already been prophesied to have either a long and unremarkable life or a short but glorious one. So if he jumps down and then turns and sails away, the entire army is guaranteed to not only win the war but also outlive a young man who will live to be old. You get free victory, and longevity for all the soldiers into the bargain. The only problem is that Achilles is literally the hardest person in the army to convince that this is a good thing.
“Hey Achilles, I know how you can easily guarantee that your side wins the war and save the lives of thousands of your friends and allies!”
“Do I get to be remembered throughout history as the greatest of all the heroes in the war even though I’m a whiny jerk? Because I like Homer’s version already.”
“No, but…hey, come back!”
Of course, winning the war isn’t that easy. The seer Calchas told the Greeks that the city would fall in the tenth year. So if they did the Iolaus thing, it’d be an army of men who can’t die attacking a city that can’t fall. That’s what you call a stalemate. But why bother besieging a city if you already know not only what the outcome will be but when it will happen? Here’s what you should do: Declare war. Then, instead of attacking the city the regular way, start telling them that Calchas says their city is going down in ten years and if they release Helen now you’ll let them live. (Evil Overlord Rule 40: If I have an unstoppable superweapon, I will use it.)
The Trojans know to take prophecy seriously. And you can’t say they wouldn’t buy it, because they totally fell for “Calchas said we needed to sail away and leave you this giant wooden horse, so we did” and that wasn’t even true. After a few raids with no Greek casualties whatever, they might start paying attention. They probably still refuse to surrender, because there’s a lot of honor in impossible last stands, but at least there are fewer casualties when the sack does happen and a less unpleasant intervening decade.
Often in Greek myths, messing with prophecies is a Bad Idea. But apparently that’s just because people keep trying to keep them from coming true, which is the opposite of this. But you’re allowed to cause the prerequisites for prophecies to be fulfilled, manipulate conditional ones into not applying, steal sacred artifacts of the gods, or even make up prophecies entirely. The gods themselves do some of those. So it’s a safe guess that they won’t mind you being clever with how they come true.
Incidentally, whoever arranged for Cassandra to be at Troy basically handed the war to the Greeks. Her prophecies have the properties of being 1) true and 2) never believed. So the Trojans end up being divinely forced to not believe important true things. Ouch. That all but guarantees that they lose any war based on weaponized prophecy. (Evil Overlord Rule 203: All crones with the ability to prophesy will be given free facelifts, permanents, manicures, and Donna Karan wardrobes. That should pretty well destroy their credibility.)
So the obvious question is, what happens if both sides are smart? If they’re both run by competent Evil Overlords wielding prophecies?
Priam hides the Palladium. On second thought, Priam doesn’t do this because a previous king did it a long time ago because it’s just that obvious. Anyway, since the city is protected as long as the Palladium is within the walls, it gets bricked up inside the walls of the innermost citadel. You’re safe as long as that’s there, and they’re not getting a chance to find or extract it without completely destroying multiple layers of walls, which would be difficult to do without violating the protection.
Agamemnon picks some volunteers to dramatically disembark first with an army behind them. Then he orders them back to the ships, making sure to keep track of which one is actually the prophetically important one. The Trojans will probably find out the reason eventually, but they won’t know which to attack first, especially if the Greeks use Odysseus’ “jump on your shield so you didn’t touch the ground” trick. (Yes, this would make the Trojan War secretly a game of The Floor Is Made Of Lava. Alas for what could have been.) And the Trojans definitely don’t get to know that the first person was actually Achilles the previous day.
Agamemnon then withdraws and sends everyone home, to come back in ten years.
Priam sends Cassandra to the enemy. Either by telling her the plan or by driving her away angry enough that she wants to betray Troy, depending on how Evil we’re talking about. When Cassandra tells the Greeks she has foreseen that Troy will fall, they give up. She can also undermine Calchas’ credibility by agreeing with him.
Ten years later, the city falls anyway. That was a pretty direct prediction, not really avoidable. But the Palladium guarantees safety for the city, so Priam just has to pick someone to surrender to. The city is then conquered completely nonviolently and still under the protection of the Palladium, fulfilling both prophecies.
The final outcome is that the Trojan War never happens and Troy stays protected. Priam isn’t king anymore, but that’s nowhere near as bad as having the city change hands the usual way. All the Greek soldiers survive at least as long as Achilles does, making this probably the only war in recorded history to have a negative death toll. And Agamemnon takes over the world with his armies of men who have been prophesied to not be killed. After all, they can’t very well fight each other like they normally would. You’ll notice that this worked out better for both sides than the original did. Moral of the story: Don’t just ignore potential instant win conditions.