Which lines in this excerpt from w. b. yeats's "an irish airman foresees his death" suggest that the speaker voluntarily chose to take part in the war?

[i know that i shall meet my fate]
[somewhere among the clouds above; ]
[those that i fight i do not hate,
those that i guard i do not love; ]
my country is kiltartan cross,
my countrymen kiltartan's poor,
[no likely end could bring them loss
or leave them happier than before.]
[nor law, nor duty bade me fight,
nor public men, nor cheering crowds,]