Did America know beforehand that Japanese have the mock up war plan to attack Pearl Harbour?

Answer 1

There was the distinct possibility that they did know hours, or even days, before hand but, at the time there was to much confusion over radio reports, the length of time it took to go through "channels" sometimes days, to get to the right people. Sometimes, lower echelon people just gave up trying thinking the Japanese wouldn't dare try the U.S.
Years earlier we'd shown our strength with out Naval excursions around the world, our might in the Pacific. It was meant to scare the Japanese and others.

No one knowledgeable in the history would want to step forward on the evidence on hand and say "yes" they knew.
It would be journalistic suicide.

Answer 2

Many militaries have war plans. Having plans and being able to carry them out are often two different things. The American military just didn't think Japan could travel that far undetected. Also American war planners had not grasped the impact air power would have yet. They knew air planes could be dangerous but hadn't understood just how dangerous. They still believed the real threat was the battleship and they knew where Japans battleships were.

Answer 3

Franklin Delano Roosevelt was advised of the possibility of such an attack, but decided not to act on it, so politically he could not be seen as an aggressor. He would rather thousands of US sailors and airmen die for his political advantage and then turn America into the saviour of the world. He had already decided to increase military spending and assisted the British as much as possible, but because of the isolationist views of congress would not commit to war until provoked. This also had the advantage that other world powers were depleted in prestige and power and could not pose a serious threat to America. FDR held private hopes the USSR would be destroyed by Hitlers wehrmacht

This enabled him to be the only person elected as President 4 times, albeit the final election was only for a short time before his death. Amendment 21 of the US constitution has since made it law for nobody to serve more than two terms as President. If it wasn't for his political megalomania and wanting to rule the most powerful nation. Imagine what could have happened if he hadn't died a month before the end of the war in Europe.

Answer 4

Yes - both Russian and British 'Intelligence' informed the American Government of the impending attack - the Americans chose to ignore that warning because they assumed (wrongly) that the Japanese were incapable of launching such an attack.