Shortly afterwards, he was sent to Hong Kong to protect British interests during civil war in China. He qualified as Italian interpreter in 1928. He was appointed adjutant of the 5th (Cinque Ports) Battalion, Royal Sussex, a Territorial Army (TA) unit, as a temporary captain on 1 May 1929, this becoming a permanent rank on 30 September 1932. In 1933 he was posted to Karachi and then to Egypt at the time of Benito Mussolini's Italian invasion of Ethiopia. It took Whistler twenty-one years after being commissioned to achieve the rank of major which he attained in August 1938, shortly before his 40th birthday. In 1938 he became adjutant of the 2nd Battalion, Royal Sussex and served in Palestine during the Arab revolt until the outbreak of the Second World War in September 1939. Unlike a large number of British commanders who were to attain high rank in the upcoming war, Whistler had not qualified for the staff colleges at either Camberley or Quetta, having twice failed the entrance examination for the former, and confided in his old Harrow and Sandhurst friend, Reginald Dorman-Smith, the younger brother of Eric Dorman-Smith, that he would end his military career in command of a battalion at most. With little prospect of advancement to higher command, "Bolo" Whistler had seriously been considering leaving the army for civilian life when war broke out.