One thing the U.S. military has added to their institutional memory since Vietnam is that the next war is rarely like the last one. For a long time, generals and their staffs tended to assume the next war would be like the last one. For centuries, that sort-of made sense. But in the last century, the rate of technological, cultural and other change has been so great that warfare has evolved in unexpected ways as well. Warfare, and wars, have been quite different, partly because of the easy availability of who did what to whom and how, on the Internet.
Having been preparing for World War 3 since World War II, Vietnam caught the American armed forces by surprise in the 1960s. After that came Grenada, Panama, Somalia, the Balkans, Haiti, Kuwait, Afghanistan and Iraq, plus a worldwide search for Islamic terrorists. Each of these "wars" was quite different, and often caught the American military unprepared.
In response, the Pentagon planners evolved a system for working out how they will cope with a wide variety of possible future wars. All this is kept very secret. Any news of these plans is guaranteed to turn into a media circus, as pundits do, well, what pundits and headline hungry media do. But for the troops, these plans are a matter of life and death. While Iraq, Afghanistan and the war on terrorism are portrayed by the media as improvised, such was not the case. There were plans, and commanders often made casual reference to these plans, and staff training exercises used to test the plans. But this was not considered news. Being unprepared was considered news. To paraphrase an old bit of journalistic wisdom, if the legend outshines the truth, print the legend. When you're dealing with secret planning, that's easy to get away with.