It was also a response to WWI. Tolkein's childhood friends were killed and shattered by that war...and why? Because in 1831, Britain agreed to be a guarantor of Belgian neutrality.
So when Gondor calls for aid... when Isildur calls the Men of the White Mountains to war... when Minas Tirith is besieged... what ought elves and men do but honour their allegiance? We now have all seen that war is obscene as cancer, but perhaps to march to death is still more fitting than to forsake our friends and break all bonds of fellowship.
Otherwise... what was the point?
But stories are still about (among other things) the choices people make, the way we see those choices, and the things we think about our own choices.