On 8 February 1915, the Australian government received a request from the
British Government of India for aerial assistance in the
campaign against the Turks in Mesopotamia. Sufficient aircrew and supporting personnel were available for only half a
flight, so the unit, the AFC's first to see active service, became known as the
Mesopotamian Half Flight. Petre was appointed the Half Flight's commanding officer and embarked for
Basra via
Bombay on 14 April, later to be joined by fellow pilots White, Merz and Lieutenant William Treloar, along with thirty-seven ground staff. In Mesopotamia, Petre was required to lead the AFC contingent in
reconnaissance and
sabotage missions, and had to deal with unreliable machines, hazardous terrain, and the threat of incarceration or death at the hands of hostile tribesmen. The obsolete aircraft supplied by the Indian Government, two
Maurice Farman Shorthorns and a
Maurice Farman Longhorn, were only capable of top speeds of 50 mph (80 km/h), and the desert wind (known as the
shamal) could reach 80 mph (129 km/h), meaning that the aircraft often made no headway or were simply blown backwards. In July, the Half Flight's equipment was augmented by two
Caudron G.3 aircraft, a marginal improvement on the Farmans, but still prone to mechanical failure. Later that month, one of the Caudrons was forced to land in enemy territory. Its crew, Merz and
William Burn, a New Zealand military officer, were never seen again; they were later reported killed by Arabs after a running gun battle over several miles. On 24 August 1915, the Half Flight was augmented by four
Martinsyde S.1s and redesignated
No. 30 Squadron,
Royal Flying Corps (RFC). The squadron moved into
Kut following the city's capture by the Allies during the
Battle of Es Sinn in September; for his part in the operation, Petre was again mentioned in despatches. Over the following two months, both Treloar and White were captured and became
prisoners of war, leaving Petre as the only pilot remaining from the original Half Flight. During the
siege of Kut between December 1915 and April 1916, he flew a series of missions using crude parachutes to airdrop grain supplies (and a
millstone for grinding), medical supplies, and equipment to the town's entrapped garrison, which included nine of his AFC mechanics. Petre was awarded the
Military Cross on 14 January 1916, and was mentioned in despatches twice more over the course of the year. In May 1916 he contracted
typhoid and was sent to India for recuperation. He transferred out of No. 30 Squadron in December, and was awarded the
Distinguished Service Order the same month. In February 1917, he was posted to France with
No. 15 Squadron RFC, Two months later his youngest brother John, a squadron commander in the
Royal Naval Air Service and a
Distinguished Service Cross recipient, was killed in a flying accident. Petre subsequently returned to England and took charge of
No. 5 Squadron AFC (also known as No. 29 Squadron RFC), a training unit for Australian
fighter pilots, particularly those destined for Palestine. He had hoped to command
No. 1 Squadron AFC in Palestine but received an adverse report concerning his leadership abilities, and the position went to Williams. Petre was discharged from the AFC as a
major on 31 January 1918, to take a commission with the RFC. In April that year, he transferred to the newly formed
Royal Air Force, establishing and commanding
No. 75 (Home Defence) Squadron. ==Later life and legacy==