Pact of the Big Three

The Pact of the Big Three was a deal made by the Big Three gods: Zeus, Poseidon, and Hades, after World War II. Because of the potential danger of the Great Prophecy, and the fact that World War II was caused by their children, they vowed to have no more children with mortals. This pact was lifted after The Second Titan War.

Examples of the Pact Being Broken

 * DSC02894.jpg broke the Pact first when he saw a TV star, Ms. Grace, who was described as a fluffy blonde starlet. With her he had a child, Thalia.
 * The second was Poseidon who fathered Percy with Sally Jackson. He tells Zeus that it was his wrongdoing so when Percy returns Zeus his Master Bolt in The Lightning Thief, Zeus allows Percy to live for the time being, but is still mistrustful of him (Zeus forgives Percy later). Percy feels that Poseidon thinks that he is a mistake, but will be proud of him.
 * Hades had sired Bianca and Nico di Angelo before the pact was made after the end of World War II. However, due to Zeus being scared of any child born of the Big Three, he tried to kill them with his lightning bolt, only managing to kill their mother (Maria di Angelo) due to Hades's interference. In order to hide his children, he hid them in the Lotus Hotel and Casino where they would age slowly and give Hades a chance to control the prophecy. Because they were born before the pact was made, Hades is the only one of his brothers who did not to break the pact. In The Lost Hero, Annabeth claims that they have all broken the Pact but Hades had the children before the pact was made.
 * While still unclear if the Roman gods had a similar pact to their Greek forms, Zeus and Hades (in the form of Jupiter and Pluto respectively) each had a child; Jason Grace and Hazel Levesque. However, Hazel would have been born before the pact was made, meaning Pluto still wouldn't have broken the pact.

Reasons for the Pact
There are many reasons why the Big Three Pact was made by Zeus, Poseidon, and Hades after World War II:
 * They wanted no more disasters or wars caused by their demigod offspring, as their children were too powerful.
 * They didn't want any demigod offspring that would be powerful enough to overpower them.
 * The oracle prophesied that a child of the Big Three gods would decide the fate of the gods, possibly for the worst.

The Roman Gods
While still unclear if the pact was made at this point in the series, the Roman gods may not have had to make the same pact as their Greek counterparts. None of the Roman campers mention any kind of limitation on children for the gods during The Son of Neptune. Also, when Percy arrives at camp, none of the other campers are shocked to see a son of Neptune like the Camp Half-Blood campers were in The Lightning Thief. At the same time, none of the other campers think it is strange that there are two children of Pluto (Nico pretends to be a son of Pluto while at Camp Jupiter) so the Pact of the Big Three may not be in effect for their Roman forms.