Complex sentence: The defences of Giarabub were at the end of a long supply line, made worse by a lack of vehicles and the Italian army lacked the mobility to maintain outposts against opposition and could deliver only a small amount of supplies by air.
Split into simpler sentences: The defences of Giarabub were at the end of a long supply line, made worse by a lack of vehicles. The army lacked the mobility to maintain outposts against opposition and only small amounts of supplies could be delivered by air.