What makes us so certain it's Grand Supercycle [III] that ended?

Because Prechter's ultra-large-scale wave count says so?  But Prechter also told us to short silver at $20, which was a poor suggestion ($45 would have been better).

It's pretty clear that the Dark Ages were a social-mood downturn at very high degree.  But the Dark Ages proper didn't stretch all the way to the Renaissance; there was a period called the High Middle Ages in between.  Prechter calls this Submillennial ONE, with Submillennial TWO the general turmoil of the 14th century; submillennial THREE is an extended third wave.

But just because third waves can extend doesn't mean they do.  Let's look at the 14th century more closely.  What we notice is that the late 14th century is characterized by the inception of positive events - the Union of Kalmar, the start of the Italian Renaissance.  But the middle 14th century in particular is rife with all sorts of wars popping up all over the place.  (Granted, the preceding few centuries had a lot of them, but wars took a long time to wage in medieval times, and if it's a Grand Supercycle [I] there's stil probably a lot of leftover lingering from the Dark Ages.)

Arbitrarily, we'll look at the year 1337.  This is not because of some Internet slang involving this particular number, but rather because the Hundred Years' War started then.  Now, wars typically don't begin at the pinnacle of an upward (grand) supercycle, but somewhat later.  So we'll put this twenty years back, in 1317 - about the time of a major famine.  (Now, said famine is probably more attributable to climate change in the form of the Little Ice Age than social mood, but...).

Backdate a Fibonacci 233 years from 1317.  Where do you end up?  The year 1084.  Now let's take a look at the Wikipedia articles for the 11th and 12th centuries.  On the page for the 11th century (year 10xx), the word "treaty" appears just three times.  That's it, three times.  On the page for the 12th century (year 11xx), by contrast, the word "treaty" appears twenty times.  Battles and wars obviously continue throughout, but they seem to become less prominent as the 12th century progresses.  The 13th century has a lot, but most of these appear to be related to the Crusades, which are something generally different and seem to be more on the lines of giving the knights and other soldier classes something to do on the grounds that they face little existential threat.  Overall if I had to live in medieval times, I'd probably pick sometime in the 1200s.

Now, what is generally thought of as Grand Supercycle [II] - the bursting of the South Sea bubble and the subsequent economic depression - ostensibly lasted about 64 years, from ~1720 to ~1784.  64 years from 1317 is 1381 - a turbulent time with peasant revolts and similar, but also a time in which changes were beginning to occur in the form of the Renaissance.  But second waves, if that is what this is, are sharp - let's estimate a nice Fibonacci 34 years, putting it at 1351.  Holy crap, that's about the time the Black Death ended!

Add the Fibonacci 233 years to 1351, putting us at... 1584?  No, too many wars ending and scientific/cultural works being done.  But 377 years therefrom is... 1728.  Uh-oh, that's pretty darn close to the South Sea Bubble bursting.  And a Fibonacci 55 years from that is 1783, just about the ostensible end of Grand Supercycle "[II]".

But the South Sea Bubble burst "early", in 1720, fifty-five years from which is 1775.  Okay, so the American Revolution didn't actually start until that year, but you can feel the percolating social mood of the downturned Supercycle (c) in the years prior, and what was that they said about wars sometimes post-dating the actual social-mood Supercycle bottom?

Now, 233 years from 1775 is... 2008.  Um... I'll just leave it there.  But look at this:

Grand Supercycle [I]:  ~233 years
Grand Supercycle [II]:  ~34 years
Grand Supercycle [III]:  ~377 years (Prechter's Grand Supercycle [I])
Grand Supercycle [IV]:  ~55 years (Prechter's Grand Supercycle [II])
Grand Supercycle [V]:  ~233 years (Prechter's Grand Supercycle [III])

In this case, I hope the bulls, or at the very least the bearish-but-not-quite-as-bearish-as-this Prechter, are right and this is "only" Grand Supercycle [IV] (or even Cycle IV of (V) of [III/V] on our way up to V to close out the Grand Supercycle).  If not, well... let's just say that a "21st century solution" to things may not exactly be as pleasant as it currently seems.  And that's not a good thing.  Not by a long shot.  This count makes Prechter seem optimistic about the near future.

(Also, if this is the case, we need to seriously reconsider the "Submillennial" degree name, as there seems to be nothing sub-millennial about it.  In such a case I recommend it be scrapped entirely.)