Konfrontasi, or Confrontation, was declared on Jan 20, 1963.
Indonesian President Sukarno, who had opposed the formation of Malaysia, which included Singapore, hoped to demoralise its people into accepting "accommodation" with Indonesia as the only way to have peace.
Indonesians were instructed to sabotage important installations and public utilities such as the Pasir Panjang Power Station, petroleum refineries in Singapore, the main water pipeline from Johor to Singapore, bridges and military installations.
When the Federation of Malaysia became official in September 1963, Indonesia broke off diplomatic relations.
After that, Indonesian saboteurs caused at least 42 explosions in Singapore, including at the water pipeline, Istana, Raffles Hotel, the Royal Air Force's Changi base, schools and houses.
Seven people were killed and more than 50 injured in the blasts.
Monday marked the anniversary of the 1965 explosion at the 10-storey Hongkong and Shanghai Bank Building, also known as MacDonald House, at Orchard Road.
At 3.07pm that day, an 11kg bomb ripped through the building, killing three civilians and injuring 33 others. The bombing was the worst attack in Singapore by Indonesian saboteurs.
The three who died were: Mrs Elizabeth (Suzie) Choo Kay Hoi, 36, private secretary to the manager of Hongkong and Shanghai Bank. She left behind six children, then aged 12 to 20.
Miss Juliet Goh Hwee Kuang, 23, assistant secretary to the manager. She and Mrs Choo were working on the mezzanine floor when the bomb went off.
Mr Mohammed Yasin Kesit, 45, driver for the Malaya Borneo Building Society. While the women were killed on the spot, he died two days later from his injuries.