Rosenthal joined the Australian Imperial Force (AIF) in August 1914 and sailed with the first convoy as lieutenant-colonel commanding the Australian 3rd Field Artillery Brigade. Rosenthal was at the Gallipoli landing on 25 April 1915. He was twice wounded at Gallipoli, the second wound causing him to be evacuated to England in August 1915. He returned to Egypt when the (AIF) was expanding and given command of the artillery of the new 4th Division and was promoted brigadier general in February 1916. He was engaged in the heavy fighting on the Somme, at Pozières and Mouquet Farm and at Ypres in Belgium. He was wounded a third time in December 1916.