History of Singapore
During World War 2, Singapore was conquered and occupied by the Japanese Empire from 1942 to 1945. When the war ended, Singapore reverted to British control, with increasing levels of self-government being granted, culminating in Singapore's merger with the Federation of Malaya to form Malaysia in 1963. However, social unrest and disputes between Singapore's ruling People's Action Party and Malaysia's Alliance Party resulted in Singapore's expulsion from Malaysia. Singapore became an independent republic on 9 August 1965.
Malaysia–Singapore border
The border changed from being an international border to a sub-national boundary (boundary of a division within a country) and vice versa several times. It became an international border after the cession of Singapore to the East India Company by Johor in 1824 as Johor was "de jure" a sovereign state. In 1914, the border became that of between two British-ruled territories when Johor became a British protectorate while Singapore remained a British crown colony.