First of all, Auden is asking to meditate on the relationship between the past, the present and the future. The poem refers to the past as "History is the operator" People should not be limited by the past and it should help them to make the future -> inspiration from the past (first part of the poem). "Tomorrow the enlarging of consciousness by diet and breathing" (plus all the lines on "Tomorrow") indicate hope that humanity will draw from the creative past instead of destruction. The final lines are saying to act now in the present, not wait for History to solve everything. Once the momen has passed, you cannot offer help nor ask for forgiveness.
Auden is also mobilizing "the nations" to get engaged in the war. And also inspire personal implication of the people of spain and europe.
Overall, this poem is ambiguous, Auden doesn't take sides which makes this poem a special and a powerful representation of the Spanish Civil War. He shows the rights and wrongs on both sides (republican and nationalist).
He also shows the contrast before, during and after the war (yesterday, today and tomorrow). Before war is the time for creation, invention and offering global universal history.
Some of the lines that are the most convincing in convincing us of the significance of Spain's Civil War are in stanza 18 -> "the menacing shapes of out fever are precise and alive..." This stanza describes bery well the horror, terror, destruction that the war has brought upon the people. The "war thoughts" have replaced the normal everyday thoughts about the "medicine ad and the brochure of winter cruises".