Complex sentence: In 1944, The Army Air Forces (AAF) faced a problem with its units in the United States, which were mostly involved with training and preparing individuals and units for deployment overseas or supplying the requirements of overseas units.
Split into simpler sentences: In 1944, The Army Air Forces (AAF) faced a problem with its units in the United States. Most of these units were involved with training and preparing individuals and units for deployment overseas or with meeting the logistics requirements of overseas units.