The son of Peleus from the other side sprang forth to meet him, like some fierce lion that the whole country-side has met to hunt and kill—at first he bodes no ill, but when some daring youth has struck him with a spear, he crouches open mouthed, his jaws foam, he roars with fury, he lashes his tail from side to side about his ribs and loins, and glares as he springs straight before him, to find out whether he is to slay, or be slain among the foremost of his foes.
~ The Iliad - Book XX (Samuel Butler Translation. p143 / v163)
Zeus calls other gods and gives them permission to enter the war on the side of their choosing. Juno (Hera), Minerva (Athena), Neptune (Poseidon), Mercury (Hermes), and Vulcan (Hephaestus) join the Greeks. Mars (Ares), Apollo, Diana (Artemis), Leto, Xanthus, and Venus (Aphrodite) support the Trojans. Apollo urges Aeneas to fight Achilles telling him that his mother (Venus) is a greater goddess than Achilles’ mother (Thetis). In the ensuing fight, when Aeneas is about to be killed, surprisingly, Neptune rescues him saying that Aeneas is not fated to die. Achilles goes on a killing spree.
In book 20 of The Iliad, Achilles is portrayed as a fierce lion charging at Aeneas and the rest of the Trojans. Homer compares this to a fierce lion that was provoked by the whole countryside, implying that Achilles was provoked by the Trojans by the manner in which they killed Patroclus.