adding to my "person of the century" question, and jumping on my own bandwagon...
again, this appears as a multi-parted question: 1)what do you think was the event of the 20th century? 2) does the event necessarily have to be a "negative" (at least at first glance) event?
there are so many monumental events within that grouping of 100 years in america alone -- however, this question is surely not limited to a single country.
some examples to choose from, t'help y'get started: sinking of the titanic, assassination of martin luther king, aids epidemic, fall of the berlin wall, tienamin square, collapse of communism, oj simpson trial (hey, you'd be surprised), introduction to cars, star wars (the movie or the defense program), e=mc squared, the great depression...
thats just a few
===
my three votes (and, to be honest, its difficult for me to say which is most important, so, my list appears in a random order)
1) the end of world war II -- from the bomb(s) dropping, to the newspaper headlines, to the famous parade in nyc (which lead to all those famous movie clips and photographs). the war itself was just an enormous event that, quite literally, involved the global scale. it was a war that seemed to lend itself to hollywood drama, with comic book-like supervillainy, and unparalleled heroism. famed battles, famed victories and victors, the future of the world was at stake with every confrontation. it solidified america's place as a strong world leader, greatly divided an all-ready weakened europe (specifically germany, obviously), set standards in development and factory work and nationalism, etc, etc... and all this lead up to an equally powerful ending, with two massively charged explosions, an honorable defiance, a relinquished stranglehold... magnificent stuff.
2) landing on the moon -- perhaps mankinds single greatest accomplishment since electricity (and, before that, perhaps all the way back to the wheel, than even fire). launching ourselves off a sphere that was, at one time, our everything. the moon landing gave us the scope that we are but 1 in 1 trillion in the expansion that is the universe. but, the symbolic act of landing on it, itself, is what proved to human kind, once and for all, that, when united, there is no task, however impossible, we can't achieve. the enormity of the event unified a country, but more importantly, a species.
3) the jfk assassination -- perhaps the turning point in america's culture. one bullet (the fatal one, however "magic") defined an entire generation, eclipsed time, and stopped 200 million hearts in an instant. it marked the end of a golden, hazy age, and dawned the age of a hazier, yet brighter, modern age of change, rebellion, revolution, faith, and hope. this event, like no other, captured the imagination, spirit, and memory of a nation, and forced each individual to chose the next moments, serving as the ultimate "wake up call," for better or for worse.