This is merely to take the advantage of the vast training ground US provides, for Singapore has next to no room for defense in depth.
And this is what everyone knows but nobody would say it out loud officially: adopting the doctrine not unlike the Israeli (back in highschool I once did posed such question during a Q&A session, and I clearly remember that I got the PR officer rather stumped right there), Singapore planned its conventional warfare scenario against Indonesia and Malaysia. While Indonesia is a simpler foe to deal with - inferior navy and airforce that already stretched to protect its vast holdings, Malaysia is a major issue because Singapore is well within artillery striking reach across the Strait of Johor, thus when the crap hits the fan Singapore will hit first, create the necessary depth in defense at Johor, hold out at least a week till US effectively intervene.
Ultimately, Singapore's defense is to make the invaders pay heavily for their risk taking, while not getting their money and blood worth - what makes Singapore valuable are the high-tech industries facilities and the major oil refineries (one of the very few in the world with capability to process Venezuela crude oil), and those most will probably be damaged/lost/render invalid during fighting.
That hold true with the kind of politicians ran those countries over the years, lesser so in these days but still, there's ongoing arms race in the region and Singapore so far managed to stay ahead with the best toys and well-trained regulars in the region; but the future would lean more towards counter-terrorism, as both Malaysia and Indonesia have active AQ cells.
