Complex sentence: Athens had been chosen to stage the inaugural modern Games because Greece was the birthplace of the Ancient Olympic Games, it was unanimously chosen as the host city during a congress organised by Coubertin in Paris, on 23 June 1894.
Split into simpler sentences: Athens had been chosen to stage the inaugural modern Games because Greece was the birthplace of the Ancient Olympic Games. It was unanimously chosen as the host city during a congress organised by Coubertin in Paris, on 23 June 1894.