Unless they use an entire army of mercenaries, the army that will be mobilised is a citizen's army. What do you think?
To me, NS is NOT negotiable. There're many reasons for this - nation building, toughening guys up, etc. And it has many flaws and some will have to pay a heavy price.
But it is one of Singapore's few guarantees that the PAPzis do not 'send in the army' when elections do not go their way.