Hong Kong, officially the Hong Kong Special Administrative Region of the People's Republic of China, is an autonomous territory south to Mainland China and east to Macao in East Asia. With around 7.2 million Hong Kongers of various nationalities in a territory of 1,104 km2, Hong Kong is the world's fourth most densely populated country or territory.Hong Kong used to be a British colony with the perpetual cession of Hong Kong Island from the Qing Empire after the First Opium War (1839–42). The colony expanded to the Kowloon Peninsula in 1860 and acquired a 99-year lease of the New Territories from 1898. Hong Kong was later occupied by Japan during the Second World War until British control resumed in 1945. The Sino-British Joint Declaration signed between the United Kingdom and China in 1984 paved way for the transfer of sovereignty of Hong Kong in 1997, when it became a special administrative region (SAR) of the People's Republic of China with a high degree of autonomy.