Pearl Harbor and 9/11 were gigantic intelligence failures that required several dozen different warnings from different sources and stations, from every branch to be ignored time and time again until the last hours before they happened.
There's no excuse, there's no level of incompetence on this level that can be justified, thus the conspiracy theories.
The result is the same either way, several thousand dead Americans and a war that lasted years, with America's adversaries (USSR, China falling to communism, in ww2, Iran, Russia, China strengthened, American popularity severely weakened due to Iraq, Afghanistan's 20 year disaster, rise of ISIS and Al Qaeda).
Either it was planned this way from the start, or the American institutions were deliberately sabotaged; warnings casually filed away on desk drawers,people dismissed and fired for warning and trying to bypass the chain of command, because someone looked at the coming storm and thought it was the perfect opportunity to start to sell raincoats.