How can I annotate the poem "Spain" by Auden, mentioning the literary techniques and relevant contextual information?

W.H. Auden's "Spain" was written in 1937, during the Spanish Civil War. Auden had an intense interest in the war and had visited Spain in order to observe what was going on; the poem takes an anti-fascist approach to the factors which caused the war and discusses what the poet observed. Later, however, Auden described the poem as inauthentic and suggested that he had never entirely espoused the politicized attitudes expressed in it.
Line-by-line is not really the most illuminating way to approach this poem. Certainly an understanding of Auden's linguistic and technical devices requires a broader overview. The parallelism, using abbreviated sentences beginning with "Yesterday," is a defining feature of the poem, with this phrase being repeated twice in each of the first six stanzas, as the poet describes the events which have led Spain into the state of war in which it currently finds itself. After this technique exhausts itself, Auden begins to use enumeratio, using a repetition of "And" to begin lines, this parallelism helping to create a sense of the enormity of the war, and the endlessness of the various painful elements which the poet has observed.
Auden is not a poet particularly wedded to figurative language, metaphors, or similes. He prefers to focus on illuminating, vivid imagery pertaining to the reality of things, and as such, the stark metaphor in this poem—"The stars are dead" in the final stanza— stands out. The brevity of the statement causes the reader to question what is meant by these stars: what do they symbolize? Arguably, the stars, at which "the animals will not look," represent the dreams of a civilization who is now "left alone with our day," forced to confront reality.
In the first stanza of this poem, Auden references several external factors contributing to the Spanish civil war. The "language of size" here represents globalization "spreading to China along the trade-routes" and bypassing Spain; the country is experiencing rapid change. "Cromlech" is a Welsh word meaning "grave": Spain is being driven into its grave by the unceasing harshness of the "counting-frame," modernity rushing too quickly into a country which has, seemingly only recently, begun to understand "the invention / of cartwheels and clocks" (stanza 2). Spain is a country that was once a hive of invention, "the bustlign world of navigators," but at the same time, it seems to remain in that past world, "the chapel built in the forest," a very Catholic country.
Auden references Spain's past in the Spanish Inquisition as he discusses "the trial of heretics among the columns of stone." Through these brief allusions, Auden creates a picture of the country's past, from its medieval origins up to "the installation of dynamos and turbines." This retrospective is brought to an abrupt close as the poet states, mid-line, "But to-day the struggle."
Spain's reluctance to face the truth of today is indicated in the fact that, in the following line, Auden returns again to the trusted parallel phrase "Yesterday." But yesterday can be no more a focus in Spain. The cumulative energy of the poem has begun to pick up, as the "poet" begs, "send me the luck of the sailor."
The Spanish Civil War was a war fought by ordinary people, and some of the ordinary people in Spain of this era were very poor indeed: "in their fireless lodgings, dropping the sheets / Of the evening paper." Auden portrays these people, informed only through the corrupt news media, waiting for "Time the refreshing river" to correct the situation for them. He refers to the external "nations" as rejecting Spain's cry for help, suggesting that because Spain had once been "yesterday," "the mover," it should be able to "build the just city" itself. Auden is here criticizing the refusal of the wider world to contribute to Spain's struggle.
Auden uses imagery to depict the vast and the disparate communities of Spain, from the "remote peninsulas," to the "aberrant fishermen's islands," and the "corrupt heart of the city." A rare simile describes how these people "have heard and migrated like gulls or the seeds of a flower" and "clung like burrs to the long expresses" of life. Spain is in part European, but it does not seem to belong here, being "soldered so crudely" to it while being, in fact, "nipped off from hot Africa." Spain does not seem to invite "the institute-face, the chain store, the ruin" or, indeed, the "greed" of other countries.
Madrid, Auden says, is "the heart" of the war. In the heart of it, Auden depicts "tenderness blossom[ing]" here, as the heat of the fighting draws tenderness and compassion from all sides, but currently "the ambulance and the sandbag" are Madrid's reality.
Towards the end of the poem, bookending the "Yesterday" parallelism of the beginning, Auden begins to muse on what might happen in the new day: "To-morrow, perhaps, the future." The final few stanzas all repeat "To-morrow," as if attempting to bring it forth, anticipating "the rediscovery of romantic loe" and other seemingly insubstantial things which are not possible in times of war. As opposed to the real bombs exploding, Auden anticipates, "the poets exploding like bombs," a metaphor which seems to suggest that the war may, at least, provide material to drive art. More important to this vision of "to-morrow" are the thoughts of people walking by the lake and bicycling—but once again, Auden pulls the reader back to the present: "But to-day, the struggle." The daydream of the future is a reverie, an interlude within the poem, but the poet does not allow us to become lost in it. It is a "makeshift consolation," and we must be returned to "to-day," a word repeated four times in the final three stanzas to emphasize the fact that, for all our daydreams, today is all we have. In the current time, "the stars are dead," and the poet, representing those he has observed in Spain, feels "left alone," unsupported by other nations and unable to see a clear bridge between "to-day" and "to-morrow."