The United States is located in the center of the North American continent, with Canada to the north and Mexico to the south. The United States extends from the Atlantic Ocean on the eastern coast of the nation to the Pacific Ocean which borders the western one, and also includes the state of Hawaii, a series of islands located in the Pacific Ocean, the state of Alaska located in the northwestern continent over the Yukon and numerous other possessions and territories.[1]The earliest known inhabitants of the modern United States territory are believed to have arrived over a period of several thousand years beginning 15,000 - 50,000 years ago crossing Beringia in Alaska. [1] [2] Concrete evidence of the settlement of these cultures in what would become the United States dates back at least 14,000 years.[2]Research has revealed much about the first Native American settlers of North America, as indicated by Cyrus Thomas.[3] Columbus's men were the first documented Old Worlders to land in United States territory when they arrived in Puerto Rico on their second voyage in 1493.[4] Juan Ponce de León, who arrived in Florida in 1513,[5] is considered the first European to land in what is now the continental United States, although some evidence suggests that John Cabot may have reached what is now New England in 1498 At the beginning the United States consisted only of the thirteen colonies, that is, states that occupied the same lands as when they were British colonies. American colonists fought the British Army in the American Revolutionary War of 1770 and issued a Declaration of Independence in 1776. Seven years later, the signing of the Treaty of Paris officially recognized independence from Great Britain.[8] In the nineteenth century, the westward expansion of the territory of the United States began, according to the belief of manifest destiny, according to which the United States would occupy the entire North American territory from east to west, from the Atlantic to the Pacific. In 1912, with the admission of Arizona to the Union, the United States achieved this goal. The outlying states of Alaska and Hawaii were both admitted in 1959. Ratified in 1788, the Constitution serves as America's supreme law in organizing government; the Supreme Court is responsible for enforcing constitutional law. Many social advances have occurred since the 19th century; these advances have been largely reflected in the Constitution. Slavery was abolished in 1865 by the Thirteenth Amendment to the United States Constitution; the following Fourteenth and Fifteenth Amendments respectively guaranteed citizenship to all persons naturalized in the United States.