punctuation - Is a comma before a conjunction optional, or old? (not talking about lists)

I have been corrected several times recently for putting a comma before a conjunction in a sentence (splitting phrases, not items in a list). To each their own style guide, but my understanding was that (using 'and' as an example):

• in the prehistoric era, the rule was to always put a comma before 'and', no matter the context


• in the modern era, there are two schools of style about 'and', commas and the final item in a list (which I am not concerned with here)


• in the modern era, you may put a comma before 'and' in a non-list to emphasise, indicate a pause, tweak meaning, etc. So that:

We will fight them on the beaches and the landing grounds.

has a slightly different meaning to

We will fight them on the beaches, and the landing grounds.

...but both are valid.

Is this comma in fact optional, or always to be discarded? Have I half-learned a (possibly out-moded) style rule without realizing it?

Answer

The purpose of punctuation is to help the reader understand the grammatical structure of a sentence. In your example, the beaches and the landing grounds are equal complements of the preposition on. A comma after beaches would suggest that they were somehow not equal, and there is no reason to suppose that that is the case.