Taipei, officially known as Taipei City, is the capital city and a special municipality of Taiwan. Sitting at the northern tip of the state, Taipei City is an enclave of the municipality of New Taipei City. It is about 25 km southwest of the northern port city Keelung. Most of the city is located on the Taipei Basin, an ancient lakebed bounded by the two relatively narrow valleys of the Keelung and Xindian rivers, which join to form the Tamsui River along the city's western border. Since 1949, Taipei has been the capital of the ROC after losing the mainland to the Communists in the Chinese Civil War.