The relationship between Achilles and Patroclus is at the center of each retelling of the Iliad.
Some tellers and translators have downplayed these two as merely best friends, noting how Patroclus repeatedly urges Achilles to marry Briseis. Other tellers and translators have depicted Achilles and Patroclus as lovers who aspired to be buried in the same tomb in order to share eternity together. They are described as foster brothers, having been raised and educated together. And some even see a metaphysical connection, where Patroclus can stand in for Achilles on the battlefield specifically because they were understood to be different aspects of a single person split into two bodies.
Regardless, Achilles and Patroclus had been closer than close since their childhoods together when Briseis enters as an outsider and latecomer to their diad. And yet, she is accepted and embraced by both of them and welcomed into a makeshift household that also includes Iphis, a prior war-bride of Patroclus. This domestic arrangement is disrupted first by Agamemnon, with his seizure of Briseis, and later by Hector, with his battlefield slaying of Patroclus.
To a surprising extent, the conflicts driving the Iliad are set within the heart of Achilles, rather than on the battlefield. This makes Achilles’s relationships with Briseis and Patroclus the keys that unlock the entire story.