First off, what is so amazing about this film is that, for the time that it was made, how modern it looks. David Lean certainly had the eye of any modern director and managed to direct a visual masterpiece at a time when many films were still being shot in black and white.
William Holden gives one of his finest performances as a cynic of warfare , citing for us the insanity and absurdity that the combatants often convey. And he hates the war, but he cannot avoid been thrown back into it again and again. We wish he could stay on the beach with his nurse lover, but he is a man destined for a tragic doom for his country, whether he wants to or not.
Alec Guiness also delivers a fine performance as a bold general whose own pride is, at the same time, his most noble quality as well as his greatest fault. He is uncompromising, yet when the Japanese submit to his demands, he begins overseeing the construction of the bridge with great esteem. Eventually, for him, the bridge becomes a manifestation of his belief of the superiority of the British Army, which he follows like a religion. And in putting all his pride into this bridge, he loses sight of even the British's own true agenda. Truly, his sense of overwhelming honor is, at the same time, his downfall in a descent to a loss of morality, and a sense of good and evil.
And yes, by the end of this film, we learn a great lesson of the horrors of war. Not only does it take the lives of many good men, but the utter failure and despair that accompany it make it an unbearable existence. And this message has only recently been re-evaluated with the also-brilliant masterpiece "Saving Private Ryan." But, keep in mind that it took forty years to regain the power that this film inspired so long ago.