Preventable Pearl Harbor??

Upon reading The Code book, which mentioned recruiting cryptanalysts for the enigma cracking team through a crossword puzzle contest, I immediately thought of my grandfather, who I consider to be a master in the art of solving the weekly newspaper puzzles. The next opportunity I had, I challenged him with the very crossword that the Government Code and Cypher School posted in the 1942 Daily Telegraph. The task was to finish the puzzle in twelve minutes.  Though my grandpa was unsuccessful, (giving up after solving about 2 of the clues), we began talking about cryptology and its role in history. Through our conversation, I learned about the Purple code, which was one of the secret methods of communication used by the Japanese during World War II. I was really surprised and intrigued when my grandfather stated that, because of this code, the United States actually knew about the attack on Pearl Harbor before it happened, and did not do anything to prevent it! The reason, my grandfather explained, was because it would give the United States an excuse to declare war. A major attack, such as the one on Pearl Harbor, would be enough to provoke nationalism and provide an incentive to enter the war! FDR, the President himself, kept American commanders in the dark about the looming attack. Also, preventing the attack would have let the Japanese know that we were able to read their codes. Over 4000 lives were stolen on the day that lives in infamy, but allowed President Roosevelt to take America into the War. Was such a loss, which could have been prevented, worth it? Did FDR do his duty as the President of the United States by lying to his people and sacrificing American lives? How would the outcome of the war have been different, had the attack on Pearl Harbor not happened?