Complex sentence: Pan Am, TWA, Cathay Pacific, Union de Transports Aériens, Canadian Pacific Air Lines and Olympic Airlines were all privately owned and considered to be flag carriers as they were often a sign of their country's presence abroad.
Split into simpler sentences: Pan Am, TWA, Cathay Pacific, Union de Transports Aériens, Canadian Pacific Air Lines and Olympic Airlines were all privately owned. Most of these were considered to be flag carriers as they were often a sign of their country's presence abroad.