"The" = definite article. Definite articles are used before specific things which are known to the speaker. So if I say I saw THE car, that means that you and I know which car I'm talking about. In this case "the battle" refers to a specific battle, the battle you know you will fight in. If you don't use "the" then that means you're ready for battle in general, ready for any battle.

As I see it, in the first case "battle" is a verb and in the second case it is a noun, because of THE article.