The idea that the Japanese could wipe out the hard core of the US surface fleet in one day or capture the Philippines in a matter of four months, let alone all the British and Dutch possessions in the Pacific - to take Singapore in a week in spite of the protection of two Battleships of what was supposed to be by far the greatest navy the world had ever seen (both quickly sunk on a single day). And all this in the context of what the Germans had done in Europe. They fought France in trenches for four years in WWI and lost, then in 1940 they take France down in six weeks. This was all unthinkable according to pre-war conventional wisdom. So things were in play, all assumptions and theories about Axis capabilities were suspect, real outcomes were up in the air as far as most people were concerned (including many military planners). I don't think there was so much confidence outside of maybe a few of the senior guys like Marshall.