Complex sentence: By the time the operation was concluded only a fraction of the known tunnel network had been destroyed and it was not until after the war that it was learnt that the system at Cu Chi actually included more than of tunnels.
Split into simpler sentences: By the time the operation was concluded, only a fraction of the known tunnel network had been destroyed. It was only after the war that it was learnt that the system at Cu Chi actually included more than of tunnels.