The capital of the United States is Washington, D.C. Since 1800, it has served as the nation’s federal capital and district, established as a separate entity from any one of the states. It is not in any of the 50 states but is instead a separate district. It became the capital as a result of political compromise and the desire for a neutral federal district.
Philadelphia was the most frequent "temporary" capital during the Revolutionary War and the early years of the government.
New York City served as the first official capital under the new U.S. Constitution (1785–1790), and it’s where George Washington was first inaugurated.