"An Irish Airman Foresees His Death"—in a poem that has "death" in the title, we expect death to be one of the major themes, right? No surprises there, we suppose. But what about death? Well, it's pretty much a certainty for the speaker of this poem. He knows he will die in battle, and yet he's not sweatin' it. At the end of the day, he's content at least his future won't be a waste of breath, just like his past was.
Silver lining time: While sad, death can often be a way to redeem oneself. The speaker describes his death as better than the "waste of breath" his life has been.
It can't be helped. Some people are just meant to die heroically. The speaker is one such person. His "fate" is somewhere in the clouds above.