In the nineteenth century, the island was considered important because it protected the passage into Keppel Harbour. Plans to fortify the island as part of the defence plan for Singapore were drawn up as early as 1827, but few fortifications actually materialised until the 1880s, when the rapid growth of the harbour led to concern over the protection of coal stocks against enemy attack. The forts built on the island were Fort Siloso, Fort Serapong, Fort Connaught and the Mount Imbiah Battery.
The western end of Pulau Blakang Mati, the place where Fort Siloso is now, used to be called sarang rimau (the tiger's den). Salusuh is a kind of herb used as a remedy in childbirth, but there is no explanation of how the fort came to be so-called, the orang laut of Kampong Kopit only knowing the place by the name of sarang rimau. By the 1930s, the island was heavily fortified and a crucial component ofFortress Singapore, and the base of the Royal Artillery.
During the Second World War, the island was a British military fortress. The British set up artillery guns in Fort Siloso that were then pointed to the south, facing the sea in expectation of a seaward Japanese assault. However, the Japanese eventually invaded and captured Singapore from the north, after having done the same to Malaya (now known as West or Peninsular Malaysia). Following the surrender of the Allied Forces on 15 February 1942, the island became a prisoner of war camp, housing Australian and British prisoners of the Japanese.
During the Japanese Occupation, under the Sook Ching Operation, Chinese men who were suspected, often arbitrarily, of being involved in anti-Japanese activities were brutally killed. The beach at Pulau Blakang Mati was one of the killing fields.
1945–1972
After the Japanese surrender in 1945 and the return of Singapore to British rule, the island became the base of the locally enlisted First Singapore Regiment of the Royal Artillery (1st SRRA)
