Up front...I am not saying that deliberately withholding information the might have saved some thousands of lives is ever morally justified. Pearl Harbor -- if so, and Coventry (more definitely known) are just two cases.

That said, it has been remarked that the anger felt by the American people to that attack carried us through final victory in the war. Without that anger, we might have made a political calculation that unconditional surrender just wasn't worth it. Certainly if Pearl Harbor had not been successful we would not have executed the Doolittle Raid. We might not have gone to make lots of escort carriers and tried the battleship based strategy - fighting the war with what we had.

Many years ago there was an interesting alternate history short story on what would have happened if the B-17's had caught the IJN and disrupted it before it got within range of Pearl. The Philippines still fall, but without bloody minded vindictiveness the American people eventually begin to ask about the cost and a peace is negotiated. The author ends it at that but it could go on from there. The atomic bomb is never completed or tested in the war. This either means that a conventional WW3 breaks out in 1950 or so, with all of the carnage from that (no nuclear threshold to keep things from going all out) or the bomb does get finished about 1948 or so. No one really knows the kind of destruction it can cause (no example of Hiroshima and Nagasaki) and we have nuclear war in the 1950's. Probably not immediately world ending then (the US, the USSR, France, the UK and eventually China did explode over 1000 bombs and more than 100 of these above ground in the 50's and 60's and we are still around) but the effects would have significantly set back civilization.

All speculation of course. In the end there was a surprise attack. The American people were incensed, and bloody minded vengeance ended the war with an example that kept the world from intercontinental warfare since then.