Six miles from earth, loosed from its dream of life, I woke to black flak and the nightmare fighters.
The bomber is flying at a very high altitude and the speaker's life before war (even life itself), left far below, feels like an illusion. Like a "dream," it slips away.
"Six miles from earth" places us kind of between earth and the heavens. This mirrors the in-between state of our speaker—asleep or awake, alive or dead?
The speaker wakes to a new reality—his "nightmare" existence in the ball turret, enemy fighters attacking.
Usually, in life it is hard to contemplate death. It seems far off and mysterious—we can only imagine it in a dream-like way. In lines 3 and 4, this is reversed: life is the dream and reality is death—the "black flak and the nightmare fighters."
The threat of death becomes more immediate, more real than life.
If we look at lines 3 and 4 together, we can see a stark contrast between the two lines in terms of imagery and language.
Reading line 3, we picture earth from a great distance looking peaceful and serene. We probably see blue sky and white clouds. Seems kind of nice, right?
The words "dream" and "life" are positive-sounding words, words that pop up in Shmoop's diary a lot when we are in a good mood. Um, no, you can't read it.
In line 4, the blue sky we may have imagined in line 3 is filled with "black flak." (check out .25, 5.30, and 12.20 to see flak in action).
And that word "nightmare" never seems to bring up good feelings. (Shmoop can still remember the first time we saw Nightmare on Elm Street—we've never been the same.)
With this contrasting imagery and language, these two lines mirror the stark contrast between life and death, or even war and peace. Pretty smooth, Randall, pretty smooth.