Answer:
There was a prophecy that said that Achilles would die at Troy. The Greek leaders are going to return Helen since they did abduct her. Achilles' parents do not want him to join the journey so they hide him on the island of Skyros. They disguise him as a girl.
Explanation:
There is also another prophecy that lets the Greeks know that they will not win the war without the help of Achilles. Now he is disguised as a travelling salesman. They visit the court with presents of jewelry and clothes and they hide weapons in them. Achilles takes the weapon and is found out - not a girl! He pulls out a sword and is recognized by the others. Achilles is the leader of the army known as the Myrmidons and he is a great fighter.