The 123rd (Royal Grenadiers) Battalion, CEF was authorized on December 22, 1915. They were based in Toronto, Ontario, and began recruiting right away, although some members had signed up for service as early as September 1915. The Battalion recruited, trained and mobilized as an infantry battalion in the Canadian Expeditionary Force during the First World War. Many senior members of the 123rd Battalion were serving with the 10th Royal Grenadiers prior to attesting in the 123rd Battalion. They had trained at home as infantry soldiers. The 10th Royal Grenadiers also contributed approximately 30% of the soldiers who attested in the 3rd Battalion (Toronto Regiment), CEF starting in September 1914.