Frankly, it's dubious to think that the USA will come to our
rescue to protect our financial- or oil-market niches. Just cuz
Singapore is run like a Inc., it doesn't mean America is also of
the same mind. Imagine the grassroots & media fallout over
there if their troops are mobilized just to protect their big
businesses (see Iraq).
Yes, the devastation of Singapore would be a major economic setback
for them, but it's not like they can't switch to contingency plans,
eg. partner ambitious Thailand in oil refinery or bankroll Hong
Kong further as a financial center. Or, for that matter, finance
the building of the Kra Canal & bypass the Straits of Malacca
altogether.
If anything, I'll say we (& only we) have consistently
attempted to keep the US military actively interested in this
region. Malaysia & Indonesia are non-aligned; the latter
becoming democractic as well. Thailand is historically neutral. The
Phillippines remained corrupted to the core. Indo-China are still
recovering from colonialist & war remnants for this generation.
America's main interest in the Asia-Pacific is in the north-east,
& have accordingly based their troops there (South Korea,
Okinawa, Taiwan if need be). Post-Clark Air Base, Singapore have
provided them an alternative refueling stop in SE Asia, but it's
likely they'll find Australia, their main ally in this part of the
world, as an adequate staging area to react to any regional
crisis.
Putting my wargaming cap on: No, we'll ultimately control our own
military destiny but, for argument's sake, IMO the countries that
will plausibly come to our aid shall be, in this order:
(a) Malaysia, but only to combine vs. hostile Indonesia,
(b) ANZAC-Britain, mainly due to Commonweath & historical ties,
but also the FPDA agreement which was conceived on the premise that
Singapore-Malaysia are indivisable in defence, never mind
sovereignty;
(c) China, assuming they ultimately have superpower designs &
wants SE Asian stability most of all, even if it takes a
Sino-sphere of influence to replace the disintegrating ASEAN;
(d) the USA, but unlikely if it's anyone other than the Chinese who
was invading, & only then likely to strike the mainland for
geopolitical control (instead of liberating us).
Thailand could slot between (b) & (c), but I dunno if they'll
take the initiative if we were warring with a fellow SE Asian
country. My speculation is they'll ally more with America
economically &, as stated earlier, activate the Kra Canal
project. Taiwan could also slot in between (b) & (c), but they
lack international legitimacy. India's revived relations with this
region is still at the infant stage. Indonesia remains largely an
unknown; we'll have to see how the new progressive political system
translates into foreign policy.