Issues of enemy technical capability and military philosophy will also affect any model used. Whilst a modeller with sufficiently high security clearance and access to the relevant data can expect to create a reasonably accurate picture of his own nation's military capacity, creating a similarly detailed picture for a potential adversary may be extremely difficult. Military information, from technical specifications of weapons systems to tactical doctrine, is high on the list of any nation's most closely guarded secrets. However, the difficulty of discovering the "unknown", when it is at least known that it exists, seems trivial compared to discovering the "unguessed". As Len Deighton famously pointed out in "Spy Story", if the enemy has an unanticipated capability (and he almost always does), it may render tactical and strategic assumptions so much nonsense. By its very nature, it is not possible to predict the direction every new advance in technology will take, and previously undreamt-of weapons systems can come as a nasty shock to the unprepared: the British introduction of the tank during World War I caused panic amongst German soldiers at Cambrai and elsewhere, and the advent of Hitler's vengeance weapons, such as the V-1 "flying bomb", caused deep concern amongst Allied high command.