Nicholson deliberately set off the explosives
I once watched a review of this film, on its DVD, where the reviewer said that Nicholson accidentally set off the bombs, as he would never intentionally destroy a bridge which he had been working on for so long. However I don't think this is the case, because in the final scene the Colonel says "what have I done", presumably realising he's been collaborating with the Japanese. Then he runs towards the detonator, before being mortally wounded by Warden's mortar shell. Unless he was planning the set off the bombs, he wouldn't run towards the detonator.
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