Yes, it is. When you combine a preposition with a noun in this way, it often becomes an adverbial phrase. For example, in the sentence 'He died in battle', the phrase 'in battle' functions as an adverb to modify the verb 'dies'. In most cases, there is no need for an article in such phrases. The commonest examples of these are phrases which you learn at the elementary level of English - for example: at home, in bed, at school, at work, by bus.

The thing that has you confused here is unrelated to prepositions. You are being confused by the difference between specific and non-specific uses of a noun. For example, consider the sentences "The battle was terrible" "Battle is terrible". The first speaks of one specific battle whereas the second speaks of all battles in general. The same is true for nouns that are used as objects of a preposition, so for sentences that start "In the battle,..." "In battle, ..." in the first case you are discussing a specific battle and in the second you are discussing battles in general.