Xenia (Greek) - in The Iliad

In The Iliad

The Trojan war described in the Iliad of Homer actually resulted from a violation of xenia. Paris, from the house of Priam of Troy, was a guest of Menelaus, king of Mycenaean Sparta, but seriously transgressed the bounds of xenia by abducting his host's wife, Helen. Therefore the Achaeans were required by duty to Zeus to avenge this transgression, which as a violation of xenia was an insult to Zeus's authority.

Diomedes and Glaucus meet in battle and before attacking, the former asks the lineage of the latter. Glaucus tells his lineage, upon which Diomedes realizes their guest-friendship. They trade armor.