Zeus saw how angry Achilles was and he was ready to fight, so he called an assembly of the Gods becuase he feared Troy would be conquered before the proper time.
There he tells the Gods they are allowed to prevent this how ever they choose.
Apollo believes if Aineias were to fight Achilleus he would be sure to win since he is the son of Aphrodite.
As Achilleus and Aineias go into combat Poseidon and Athene watch and question whether they should intervene or not. Poseidon later figures he should save Aineias becasue it isn't fair that Apollo tricked him into fighting Achilleus.