If Malaysia invades, Singapore will win.

Malaysia doesn’t have the ability to launch large scale invasions. It announced in 2013 (amid the backdrop of Chinese assertiveness in the SCS) that it wanted to establish a marine corps to have that ability but no further news was heard since. Singapore is close enough to Malaysia that you don’t need a marine corps to invade the island, however a successful invasion still depends largely on attaining air superiority or at least air parity over the beachhead.

Malaysia has a mix of relatively modern SU-30MKM, F-18 and MiG-29N fighter jets with a respectable arsenal of both Russian and American modern missiles to support an invasion force of assault boats and landing craft. However they are all stationed in the northern region of Malaysia and Singapore has airborne early warning and control (AEW&C) aircrafts in addition to reconnaissance UAV assets constantly patrolling its airspace in addition to ground radars (and possibly satellites) to prevent the possibility of being sneaked upon by hostile aircrafts and massed military assets at its doorstep.

Further, there are US aircraft carriers calling Singapore’s ports frequently and even the US Navy’s newest Littoral Combat Ships are stationed there on a long term basis. You don’t go and rob a restaurant when there’s a heavyweight wrestler dining there and who’s friends with the owner.

Conversely if Singapore invades, Malaysia will win.

It’s an open secret that Singapore has the capability and specifically trains for an assault into Johor with a so-called Mersing Line as a target to reach and hold in order to have the water pumping stations in Skudai and Kota Tinggi well secured and beyond range of weapons such as the Astros II MLRS or 155mm howitzers.

Malaysia may not be as well equipped militarily as Singapore but it has the ability to trade space for time. It knows it can’t win a war head-on with the Singapore Armed Forces and hence wouldn’t engage directly.

Instead it will engage in guerilla warfare and jungle warfare with Singapore’s conscript army who are more familiar with the layout of their shopping malls rather than the Malaysian jungles. A conscript army engaged externally also means that Singapore’s economy will ground to a halt due to missing manpower if sanctions by other countries doesn’t cripple the country in two weeks to a month’s time.

So if Singapore were actually on the defensive and doesn’t go beyond the Mersing Line, it’s logistics line should be manageable as per drawer plans but the further it goes beyond it the more it will be stretched and be vulnerable to guerrilla tactics by Malaysia. Not to mention it’ll have to deal with more captured ‘POW’ and civilians. They need to be kept under control, fed and taken care of. It’ll be a huge logistical burden on Singapore’s manpower and a burden that their premium war-fighting equipment isn’t actually designed to deal with.

Now if purely for the sake of this question the two armies goes for a mindless all out battle, then Singapore with more personnel that it can muster and more and better critical equipment will win.