THAT WAS THE MILLENIUM THAT WAS by John Scalzi

the Enigma machine until they got actual deciphered messages. 
    In 1939, realizing Poland was about to be sliced up like an Easter ham (they had the German's messages, after all), the Poles set up a secret meeting with a Brits and handed over all their research on Enigma up to that point. The Brits were dumbfounded, to put it mildly. Did they let Rejewski, Rózycki, and Zygalski on to Ultra project? Of course not. They were foreigners , you see. They had enough problems sharing information with the Americans . 
    (Who, incidentally, were busy cracking a code of their own: "Purple," an Enigma-like code used by the Japanese. It was no small task -- the lead researcher on Purple suffered a total nervous breakdown -- but it yielded very positive results. Thanks to cracking Purple, an American fighter planes "just happened" to shoot down a plane carrying Isoroku Yamamoto, commander-in-chief of Japan's naval forces. He was the guy who suggested attacking Pearl Harbor, you know, so there was probably very few tears shed over what was, in fact, a bald-faced assassination by aeroplane.)
    It's not an exaggeration to say that the need to crack the Enigma code expanded human knowledge considerably. Much of this expansion took place in the rarified field of mathematics -- by the time of WWII, cryptanalysis was indistinguishable from higher-order math, and today it's even more so -- but other fields also got their share. The first programmable computer was not constructed in the United States after the war as is generally presumed, but in Bletchley Park, home of the Ultra project. The computer, called "Colossus" (because it was) was designed to crack codes quicker than any human could. You're reading this on the spiritual descendent of that first computer, "spiritual" because the machine, secret during the war, was destroyed just as secretly afterwards -- the Brits were nothing if not paranoid, and by extension, thorough in covering their sneaky little tracks. The world didn't find out about Enigma or Ultra until the 1970s. At which time, the Argentine air was filled with the sound of former Nazis smacking their foreheads in aggravation.
    As mighty an intellectual feat as cracking the Enigma and Purple codes were, the tale is also an example of how when it comes down to it, people with big brains often have to rely on people with teeny brains making really dumb mistakes. The Enigma code was broken partially because German army soldiers , confident the code was invincible, got sloppy and used simple "initial" codes -- a three letter code at the beginning of a transmission that allowed the guy at the other end to "tune" his machine to receive the message -- that allowed the Brits a window of opportunity (The German navy was more circumspect with codes and who sent messages -- as a result, the naval codes were cracked years later than the army codes). It's proof that the biggest problem with any perfect system is the imperfect humans who use it. 

Best Mass Hysteria of the Millennium.
    The Death of Rudolph Valentino. Because it was a "Chick Thing," and so was hysteria -- or at least it was assumed to be.
    I mean this literally. The word "hysteria" is directly related to hystera , the Greek word for "uterus." Seems the Greeks (who despite their large, meaty brains, had this penchant for presenting theories without the observational data to back them up) believed that hysteria was a condition of mental agitation brought on by "vapors" from the uterus. Show me a freaked-out woman, said the Greeks, and I'll show you someone with a belching womb.
    You think someone would have figured this one out eventually, but, eh, no. Well into the 19th century, "hysteria" was frequently diagnosed in women, and often the treatment was, shall we say, rather intimate. It involved "massaging" a particular nerve-filled area on a woman's body to "calm" her (men, if you can't figure out which area I'm talking about, you now know why you're so often alone on a