THE 1965 bombing of MacDonald House was targeted at civilians, and was clearly against international law, Foreign Minister K. Shanmugam said on Wednesday.
When Indonesian marines Osman Mohamed Ali and Harun Said planted the bomb in the Orchard Road building on March 10 that year, they were not in uniform, but posing as civilians.
The explosion killed three civilians and injured 33 others.
Mr Shanmugam said the attack was part of a "campaign of terror" that was contrary to the laws of war, adding that there was "nothing subjective about Geneva Conventions".
Under the conventions - a set of international treaties and protocols that lay down the rules for the humanitarian treatment of people during conflict - attacks against civilians are prohibited.
Some commentators recently said the two men were merely following military orders when they carried out the attack.
They argued that it was thus all right for the duo to be considered "heroes". The two men were executed in Singapore, but were viewed as heroes in Indonesia and given a full military burial there.
Mr Shanmugam, however, said there were "no shades of grey" in what they did.
"If it happens now, if people plant bombs to kill civilians, historians won't be debating on how to characterise it," he said.
The bombing was part of a spate of attacks carried out in Singapore, during which schools and other civilian institutions were also targeted.
It took place during the Confrontation that Indonesia's then President Sukarno pursued against Malaysia, which included Singapore at that time.
Mr Shanmugam noted that the two men had been tried in court in Singapore and were sentenced to death for their crime.
The case had also gone all the way up to the Privy Council in London, and Britain's court of appeal had upheld the decision of the Singapore courts, and found the two men guilty too.
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