What brings Achillies back into the fight?

Achilles returns to the fight primarily due to the death of his close friend Patroclus, who was killed by Hector while wearing Achilles' armor. This loss ignites a deep sense of grief and rage within him, compelling him to seek revenge. Additionally, Achilles is motivated by a desire to restore his honor, which he feels has been tarnished by his earlier withdrawal from battle. His re-entry shifts the tide of the war, showcasing the depth of his emotional connection to Patroclus and the importance of honor in his warrior ethos.