Unanticipated Challenges Quotes
Although most listeners had to spend some time parsing this famous quote, it actually was not a new notion. See Copernicus in linked quotes.
He was reacting to the Japanese Defense of Okinawa which was failing on every front. When Japan started the war with America, its leaders were not at all convinced that they could drive America from the Pacific, but they were convinced that America would not spend the men and material to invade Japanese territory and that they would be able to negotiate a peace at some point if the price of invasion was too high. That assumption was brought into doubt.
While this has been recognized in politics, it is a factor in scientific research as well.
His catastrophic failure however was when he supported the Neocon ideology that Iraq had a nuclear bomb about to be deployed and urged a preventative war invading Iraq. Not only was there no bomb ready for deployment, there was net even a nuclear research or development program to do anything nuclear.