In November 1900 Costello was appointed
adjutant of his regiment, and on 19 November 1901 he was promoted
Captain in the
Indian Staff Corps. He then worked as a recruiting officer for several years before taking part in the
Mohmand operations of 1908. He was promoted
Major in 1910. In 1913 he entered the
Indian Staff College at
Quetta and graduated just before the outbreak of the
First World War in 1914, when he rejoined his regiment as
second-in-command. The regiment was soon sent to
Mesopotamia as part of the
17th (Ahmednagar) Brigade of the
6th (Poona) Division and Costello remained there for the rest of the war. He was promoted
Brevet Lieutenant-Colonel in June 1916, was awarded the
Distinguished Service Order (DSO) in 1917 and appointed
Companion of the Order of St Michael and St George (CMG) in 1918. In May 1918 he took command of the
12th Indian Brigade and he received a substantive promotion to Lieutenant-Colonel in September 1918. He was mentioned five times in dispatches during the war and also received the French
Croix de Guerre. In June 1919 he was promoted Brevet
Colonel and was joint commander of the Indian contingent at the Peace March in
London, for which he was appointed
Commander of the Royal Victorian Order (CVO) in the 1920 New Year Honours. He was promoted substantive Colonel in March 1920, although he had held the acting appointment of
Brigadier-General since 1918. From May to December 1920 he commanded the
8th (Jullundur) Brigade in the
3rd (Lahore) Division. In March 1921 he went to
Palestine as temporary commander of the
Palestine Defence Force and remained there to command a brigade in 1922. He retired in October 1923 and became Director of Military Studies at the
University of Cambridge. ==Family==