The South African poet Roy Campbell really did fight in Spain, though as Thomas P. McDonnell puts it, "Liberal poets and academicians of the thirties and forties have never forgiven Roy Campbell for his robust participation on what they presumed was the 'wrong side' of the Spanish Civil War."
That having been said, Campbell not only participated, but in the course of events rescued the Carmelite archives of Toledo from destruction, a detail he omits from his poem about the siege there.
Toledo, July 1936
Toledo, when I saw you die
And heard the roof of Carmel crash,
A spread-winged phoenix from its ash
The Cross remained against the sky!
With horns of flame and haggard eye
The mountain vomited with blood,
A thousand corpses down the flood
Were rolled gesticulating by,
And high above the roaring shells
I heard the silence of your bells
Who've left these broken stones behind
Above the years to make your home,
And burn, with Athens and with Rome,
A sacred city of the mind.
Hat tip to A Literary Blog of Twentieth-Century and Beyond Poetry in English for the transcription of this poem.