Japan invaded Hong Kong on December 8, 1941, and the colony fell by December 25. Following several years of a brutal occupation that greatly reduced Hong Kong's population, the colony once again began to grow and expand in the post-war years, in part due to massive immigration from "Mainland China." By 1997, Hong Kong was unquestionably a major metropolis and financial center, and it was in that year that the British transferred sovereignty of Hong Kong back to China. Under the "One Country, Two Systems" principle that underlay the agreement between China and the U.K., Hong Kong's capitalist system and way of life is to remain unchanged until 2047.