Nyorai kupfuurira kweiyi ndima - Allen akagadzwa muna 1913 uye akabatana neAustralia Imperial Force (AIF) musi wa24 Chikumi 1915. Allen akaenda kuEgypt nevashandi ve13th Battalion muna August. Muna March 1916 akakwidziridzwa kuva kaputeni uye akapiwa basa mu45th Battalion. Akasvika muFrance musi wa 8 Chikumi 1916 uye akarwa muhondo yePozières muna Nyamavhuvhu. Allen akaramba ari pamberi paSomme muna 1917 uye akatungamirira varume vake kuburikidza neHondo yeMessines muna June. Kutungamirira kwake pakati pokurasikirwa kukuru kwakaita kuti Allen apiwe Distinguished Service Order uye kukwidziridzwa kuva mukuru muna July. Akaramba achitungamirira varume muhondo, paHondo yeDernancourt muna Kubvumbi 1918, uyezve semutevedzeri wemutungamiri, anotarisira 48th Battalion, paHondo yeMonument Wood
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