Complex sentence: Following a flood in Olfus river the military base was closed down but the airport remained in operation and was used in WWII at the end of which its buildings were torn down and moved to the Keflavik airport.
Split into simpler sentences: Following a flood in Olfus river, the military base was closed down but the airport remained in operation. After the war, the buildings were torn down and moved to the Keflavik airport.