Onela, son of Ongentheow, is the king of the Swedes, the rival tribe to the Geats. In an attempt to forge alliances and prevent blood feuds, Onela marries a Danish princess, the daughter of Halfdane and sister of Hrothgar. However, pretty much everyone in Beowulf recognizes that this attempt at "peace weaving"—preventing war through inter-tribal marriage—is doomed to failure.
Eventually, war will break out again between the Swedes and Danes, and between the Swedes and Geats. Despite this fact, the narrator of the poem shows respect for Onela's prowess in battle and strength as a king. Even Onela doesn't dare to attack the Geats while Beowulf is king, but, after Beowulf's death, the Geats know Onela will come sweeping down on them again.