 Pashure pokunge Britain yazivisa hondo kuGermany, Callaghan akabatana neAustralia Imperial Force AIF uye akaendeswa kuMiddle East ne2nd Battery ye1st Field Artillery Brigade. Akatora rutivi mukumhara kweGallipolimusi wa25 April 1915. Akapiwa Distinguished Service Order nokuda kwezviito zvake muna July, apo akafambira mberi nevashandi vepfuti kuenda kumakomba eTurkey kubva kwaakatumira mishumo kudzokera kubhatiri rake. Akabviswa muna Zvita 1915, pamwe nevamwe vese veAustralia neNew Zealand Army Corps, kubva kuGallipoli kuenda kuEgypt ndokuendeswa ku5th Divisional Artillery. Muna March 1916, akaitwa mutungamiri we25th Howitzer Battery uye akasimudzirwa kuva mukuru. 