Those that I fight I do not hate Those that I guard I do not love; (3-4)
These lines are almost identical (with the exception of a few words). Those the speaker fights and those he protects are almost the same thing. This means war is kind of pointless, and there are no good guys or bad guys.
No likely end could bring them loss Or leave them happier than before. (7-8)
We expect the people of Kiltartan to be affected by the war, even though they're far away from the front lines. Strangely, it doesn't seem like the war will change their lives one bit—"no likely end [to the war, to my life] could bring them loss." Hmm, sounds like some wishful thinking to us. Maybe the speaker is being ironic…
Nor law, nor duty bade me fight, Nor public man, nor cheering crowds (9-10)
All parts of society are responsible for war: the law, public officials, even citizens themselves (cheering crowds). The speaker is miraculously impervious to their combined influence and, as he will shortly tell us, joined the war for his own reasons. What a strong dude.
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 behind In 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.