Complex sentence: In 1944 the Germans invaders placed explosives in the tunnel to be able to destroy it at a moment's notice if needed but that never happened, probably due to the sabotage of the detonators (the overhead lines) by the Dutch resistance.
Split into simpler sentences: In 1944 the Germans invaders placed explosives to be able to destroy the tunnel quickly. But that never happened, probably due to the sabotage of the detonators (the overhead lines) by the Dutch resistance.