The relation between China and Vietnam also declined following reunification in 1975, with Vietnam siding with the Soviet Union against China in the Chinese-Soviet split. Tensions peaked when Vietnam and Cambodia started a war, Cambodia lead by Pol Pot being a Chinese ally, resulting in a Chinese invasion of Vietnam in 1979. In 1978-79, some 450,000 ethnic Chinese left Vietnam by boat as refugees (many officially encouraged and assisted) or were expelled across the land border with China. In recent years the government has performed an about turn and is encouraging overseas Hoa to return and invest, but the ethnic Chinese population has been in continuous decline since the 1970s due to assimilation and low birth rates.