Yes, the UN is only a governing body. Nations have time and
again ignored its demands, most recently the unilateral moves by
the US to invade Iraq. Look what happened to the US now. Bogged
down in Iraq, with little or no Allies willing to support its
operations and grounds that is conductive for fostering militancy
instead of democracy. Such is the folly of not following the UN's
advice.
Also, the Straits of Malacca is an important shipping route for
many countries, and the life-line of China, Japan, South Korea,
Phillipines, Australia, New Zealand, and many other countries that
import and export goods. In an event of a confrontation or war, it
would make sense for the major powers and developed nations to
stabilize the area, and the best option would be to maintain the
status quo. Thus they will send in military forces to keep the
peace in the region.
Yes, if indonesia manages to mobilize all its males to fight they
will have at least 100m, since their population has some 230million
(afterall they are the World's largest muslim nation). However,
look at its borders. Who are its immediate neighbours? Will they
keep a blind eye if their own economy is going to be
destroyed?
Secondly, even if there is no external support, do you think they
can launch an invasion with 100m people. Think of all the logistics
involved. Do you think they are landing on sampan boats? The
logistics planning would be on a scale bigger than D-Day. Even the
World's superpower (US) can only support an invasion scale the size
of their marines.
Thirdly, Indonesia is not truely united. There are many tribesman
and local customs that differs markly from Jakarta. Remember Aceh?
It wasnt until the Tsunami that the 2 sides are willing to make
peace for posperity. Within its 10000 islands, how many has their
utmost allegance to Jakarta? Using propaganda in the right place
will see Indonesia having internal strife.
Using an ICBM on Indonesia is an easy solution, no doubt. But once
launched, have you thought of the consequences? We will lose all
our legimicity and morality. Who will even support such a country
in this case?
Furthermore, how damaging would it be to mother Earth? Those large
lands in Indonesia houses large swarthes of Rainforest and Arable
land. Do you think we have enough land to grow all our food? The
prices of flour has rose precisely due to the drought in Australia
and low yield in other Wheat producing countries. Land would make
better use by either farming it to feed more people or retaining
its rainforest to check the growth of carbon dioxide.
Launching a land invasion against Pulau Tekong?! That's pretty
laughable. Can they reach the island first? They'd better make sure
they can avoid being destroyed en-route by our navy from Changi
base. And the Air Force will be either engaged with the Indonesia
Air Force or Bombing Indonesia's Landing Ships. Unless you count
Malaysia in.'
If that is the case, then the odds will be stacked against us in
terms of land forces. The Malaysian Army can move down South
against us while the Indonesian take its time to move up
North.
In this case, HE cruise missiles will do the trick. Or precision
bombs from the F-15SG. Strike all important military buildings on
the surface. And jam radar communications. Implement Blackouts in
Singapore. Stop all out going travel. As the strategem goes: "Then
there is no path to retreat, people will hard to their hardest and
therefore lives." We will likely hold out in the hope that foreign
aid reaches Singapore soon. The Navy will change tack, switching to
Anti-submarine warfare. By sinking supply ships of their navy, we
can reduce the effectiveness of the Indonesian Navy. The Surface
Navy can also embark on a trip to mine the Straits south of
Singapore (aka Riau Islands) while our Air Force makes desperate
defence of Singapore's Airspace.
Of course, what I've said are all theoretical and I am at best,
"taking about military strategies on paper". 
