Hi guys, thanks for all your replies and useful information! I feel much more enlightened now.

One of the reasons I asked this question was because of an earlier discussion about how effective Roman legions would have been on a Medieval battlefield. I initially argued that they would have been largely ineffective; after all, hadn't they been long outdated by that time? I assumed (rather naively) that heavy cavalry had been the downfall of the legionaire system of war. Naturally, this led me to the conclusion that the weaknesses that had led to the legion's downfall at the twilight of the Western Roman empire would translate over to the Medieval battlefield (only more so, since Western knights were among the strongest heavy cavalry ever devised). Thus, even though legionaires were probably superior to most Medieval infantry fielded, they would still be ridden down by the dominant force on the battlefield, the knights - or so I thought.

After reading these posts, it seems that I was clearly wrong. With the fall of the Roman empire and the descent of Europe into feudalism, many of the earlier advancements in civilization and technology were lost or forgotten, and military technology took a big step backward as well. The new ways of war that emerged appear to have been dictated more by social and economic conditions than by military effectiveness, and were, in many ways, inferior to the Roman military system. Deprived of the stability supplied by Roman civilization, individual settlements had to find ways to defend themselves against bands of marauding barbarians. Military forces were smaller, less disciplined, less well-equipped, and less well-supported logistically. Perhaps they were more suited to dealing with fast-moving raiders than to fighting large-scale wars. However, there appears to be no reason to think that the legionaires of old would have been any less effective on the Medieval battlefield than they had been in their own era. As they had done in the past, they probably would have found a way to deal with the threat of heavy cavalry, and would have outclassed almost any infantry opposing them.

Over time, in fact, it appears that many lessons from antiquity were eventually rediscovered or relearned. For example, the old Greek phalanx was thought to have been made obsolete by the advent of the Roman legion. However, it was resurrected multiple times in the form of pikemen who proved very effective. Examples include the Scottish schiltron and formations of Swiss pikemen, who had the flexibility to fight on uneven ground.

The lesson in all of this seems to be that newer is not necessarily better. We cannot assume that just because a practice or idea has fallen out of use (or out of style) that it is wrong or inferior. Rather, we must examine the reasons behind its drop in popularity, and judge it by its own merits rather than by its supporters (or lack thereof). Those who have gone before us were not any less intelligent than we; our age is an age like any other, with its own common errors and misconceptions.