Imagine your shock at waking up one morning to a fleet of enormous, otherworldly craft looming over you. And when bizarre aliens begin to emerge-speaking strange gibberish-your heart races even faster. Similar fears may have gripped New World inhabitants when diverse civilizations-separated by a vast ocean-first met. American natives once knew nothing of towering ships, galloping horses, thundering guns, or smallpox. From 1492 onward, however, waves...