why did the battle ensure?beowulf question?

I'm not sure what you mean by "why did the battle ensure"?

ensure means

1.Make certain that (something) shall occur or be the case.

2.Make certain of obtaining or providing (something).

So I'm thinking you meant "what did the battle ensure?"

Basically it meant that the Dragon will never bother the Geats again. Also it is decided that no man shall ever have the treasure for which Beowulf fought. Wiglaf and Beowulf's thanes toss the dragon's body into the sea. They place the treasure inside a mound with Beowulf's body and mourn for "the ablest of all world-kings."