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,]

The first part when he says he "shall meet my fate" could allude to a choice of joining. This could allude to an acceptance of fate sort of standard and that he made his decision. Another is the last two lines when he says "Nor law ... nor cheering crowds." This quotation shows that the government, the duty to his country, the men around him, and those at home cheering them on did not persuade him to fight. In the second last line this is known when he states "nor law nor duty bade me fight".