After the eight to nine years of Greek fighting the Trojans they started to figure out that part of the reason that the Trojans were so indistructible is that they had cities around them helping them and supporting them. Once the Greeks found this out, instead of sending all their troops to Troy they would send them to the surrounding cities that were helping Troy and that helped to weaken the Trojan force. Troy said that they would not fall unless:
a) Pyrrhus, Achilles' son, fought in the war,
b) The bow and arrows of Hercules were used by the Greeks against the Trojans,
c) The remains of Pelops, the famous Eleian hero, were brought to Troy, and
d) The Palladium, a statue of Athena, was stolen from Troy.