Yeats' line refers to the reason why the airman does what he does. In a time period where so many believed different reasons for war, Yeats undercuts all of them with this line. The Irish Airman fights in this war and is in the warfighting condition he is in because of his desire to fly. That is the sum total for why he is there. It is not out of love of "law" or "duty," nor is itfor "public men" and "cheering crowds." There is little nationalism or imperialism at play here, and there is no spiritual reason for the Irish Airman's participation in the war. Rather it is because the airman holds "A lonely impulse of delight," a love of flying or zeal towards a particular specialized craft that no one else shares and few others understand. This solitary purpose of fighting, at a moment where death is realized, is what helps underscores the futility of the war effort and casts a rather painful shadow on the death of so many in World War I. In being able to "foresee" his own death, this revelation becomes critical and something that makes the poem seem both an example of Modernist theory as well as predating Postmodernism, simultaneously.