I would like to know why the phrase "Killed in battle" is grammatically correct. Why does one not need to put the article "a" there?
A lot of nouns can be used two ways:
As a concrete noun, illustrating a specific instance of a concept. These appear as count nouns. For example:
- a fierce battle
- an ancient culture
- three deaths
- a college education
- perpetrating a great evil
- a disturbing experience
- a close friendship
- my first love
- a profound melancholy
- a rare opportunity
- an uneasy peace
- an unsafe speed
- a good time
- the great war
As an abstract noun, illustrating the concept in general. These appear as mass nouns. For example:
- died in battle
- promoting culture and the arts
- avoiding death at all costs
- believe in compulsory education
- all that is necessary for evil to triumph
- a job that requires experience
- valuing friendship above all else
- love is all that matters
- there is melancholy in the wind and sorrow in the grass
- opportunity is a rare thing
- seeking peace
- speed is a scalar quantity
- time waits for no man
- war is a racket
I used the same words in both sets. (I apologize for the clumsy choice of examples--I picked whatever came to mind first. Some of them are quotes or collocations.) In the first set, they were count nouns, so they needed determiners (a category which includes not only articles such as a, but also words like my and three). In the second set, the same words didn't need determiners.
Let me look at the examples I chose for your word, battle, so I can go into more detail:
- a fierce battle: This describes a single battle. This particular battle was fierce. In this phrase, battle is a concrete noun because it describes a specific instance of the concept of "battle". For this reason, it needs a determiner.
- died in battle: This describes the concept of battle in general, so it's an abstract noun. It doesn't describe the particular battle in which they died. All we know is that they were fighting when they died. So, this doesn't need a determiner.
As you can see, both are grammatical.