What the main theme of the poem?

The poem's main theme would likely be Yeat's attention to Irish history. This specific poem laments the suffering and disempowerment of Ireland's people under British control, and Yeats addresses the problem by describing the life and death of a single individual—a young man fighting for the British military in World War I. In fact, the poem originally responded to the death of Robert Gregory, the son of Yeats's friend Lady Augusta Gregory. Through the story of this doomed airman, Yeats discusses the injustice of young Irishmen fighting in a war from which they will not benefit. He also suggests that there is a class-based and regional element to the oppression of the Irish: the speaker specifically mentions being from a rural area in western Ireland, and mentions his affinity for the poor residents of the area. These poor rural people, the poem suggests, are especially unlikely to benefit at all from this war, even as they make sacrifices to fight in it.