Before European contact, indigenous peoples from Siberia migrated to North America across the Bering Strait beginning 16,000 years ago. Their populations expanded throughout the Americas, reaching an estimated 50 million inhabitants prior to European arrival. Meanwhile in Europe, the Roman Empire united much of the continent but later weakened and fell, leaving a power vacuum. In the 15th century, Spain funded Columbus' expedition seeking a route to Asia but he instead made landfall in the Americas. Over subsequent decades, the powerful Aztec and Inca Empires were conquered with devastating consequences for indigenous populations. European powers raced to claim territory in the Americas, though vast areas remained unsettled for centuries.