Complex sentence: Brett was posted to Kelly Field, Texas, in December 1918, where he commanded the Aviation General Supply Depot until February 1919, when he became the maintenance and supply officer at the Air Service Flying School.
Split into simpler sentences: Brett was posted to Kelly Field, Texas, in December 1918, where he commanded the Aviation General Supply Depot. In February 1919, he became the maintenance and supply officer at the Air Service Flying School.