And Singapore's most important military contingency is a war with Malaysia - which it has a defence treaty with?
No one else has the capacity to easily invade.
Yes, after the end of Vietnam war in 1970s, 5 Commonwealth countries - Singapore, Malaysia, UK, Australia and New Zealand formed a military allies by signing a "Five Power Defense Arrangement" treaty. The aim was to to defend Singapore and Malaysia if Vietnam and its communist bloc allies decided to go south attacking Cambodia, Thailand, Malaysia and Singapore.
Time has changed and Vietnam is no longer considered a threat, and UK has lost much of its military power. So the biggest threat to SIngapore is ironically from one of its allies because the Malay chauvinist dreaming of uniting Malaysia, Singapore and Indonesia, into an Islamic Malay Nusantara nation.
However, this view is shared among very small number of people thus it is unlikely to happen now and in near future.