A lonely impulse of delight Drove to this tumult in the clouds; (11-12)
"Tumult" is a strange way to characterize war in the air. Planes shooting at each other is definitely more than a tumult. The speaker avoids describing war as the horrible thing it really is, but why he does so is an open question.
The years to come seemed waste of breath, A waste of breath the years behindIn balance with this life, this death (14-16)
Compared to the soldier's life in the war ("this life, this death"), everything else is meaningless. This is at once a statement about dying for a cause, and yet profoundly sad. How could the speaker's prior and future life be a waste of breath? Dying in the clouds in a senseless war seems like the real waste.