The Tinderbox: Europe—all of it. By the dawn of the 20th century, the European powers were involved in so many treaties and counter-treaties, it's a wonder one of the countries didn't declare war on itself.
The Lit Match: One wrong turn.
June 28, 1914, was just one of those days "¦ that lives in infamy. That was the day Archduke Franz Ferdinand, the less-than-popular heir to the even-less-popular Austro-Hungarian Empire, chose to go on a processional drive through Sarajevo. Surprisingly, this is not the great mistake we're writing about.
Actually, the first few days of the archduke's visit to Bosnia went pretty well—particularly considering that nationalists from neighboring Serbia had marked him for death. He even managed to survive an assassination attempt in Sarajevo by successfully playing Hot Potato with a lit bomb that was thrown at his convertible. The bomb exploded in the street, and the archduke continued about his business. Unfortunately, he let his guard down a bit too soon.