WI Singapore and its neighbor Malaya were not deprived of men and material before the war against Japan, in order to protect against German aggression? I have read that Singapore, in particular, was promised much more than they received, including submarines. How would that have affected the WWII Pacific front? With a strong British presence in Southeast Asia, what would Japanese tactics and strategies have looked like?
Had land and air forces in Malaya been up to what was prerequisited in Plan Matador (appr. 50 btn and 400-500 fighters), and as the Imperial General Staff proposed in 1941, but was refused by Churchill, then the Japanese would have been in a very difficult position.
Had the British invaded southern Thailand (Isthmus of Kra) at the first indications of Japanese activity, I would call a succesfull Japanese invasion of Malaya nigh-on impossible. The British commander in the far east (Brooke-Popham) had been given authority to do just so in early december, but did not dare use it although he recieved several intel.reports on Japanese fleets approaching.
But even if not taking up the favourable posititions on the isthmus, a full size and regular British/Commonwealth force could not have been repeatedly outflanked like in OTL. But the campaign did show that when British/CW regulars met ditto Japanese frontally the first usually were the best. Flanking landings on the east or north coast would not have major influence on the west coast, as those parts were parctically separated and the N and E parts with very little infrastructure. The W part however had the most dense roadnet outside Europe and N.America and was only accessible from the north through the isthmus of Kra.
But even if the Japanese manage to get a foothold in the north, the drive south will at best be very slow. That will give the British plenty of time to send and deploy reinforcements. For instance the regular 18th Division, which in OTL only arrived in time to be captured, will have time to arrive and deploy in good order. The forces in OTL used to invade Vichy French Madagascar and Syria in early April can pose a serious flak threat to the Japanese, as the Madagascar invasion involved landing capacity for several brigades.
The Japanese OTOH did not have the logistic capacity to deploy significantly more forces in the offensive.
The reinforcements to bring Malaya/Singapore up to Matador standard before the Japanese attack would have had to main sources: The Mediterranean offensives of 1941 and the offensive fighter sweeps over Framce. But as neither were succesful, let alone decisive, that ought not change the great outcome there. Not even downscaling the anti-convoy efforts from Malta should be decisive, as the main Axis problem until much later not was getting supplies across the sea, but unloading them in North Africa and getting them from the ports to the front.
In a protracted Malayan campaign it would be interesting if the British would implement the Mediterranean tactic of operating at night radar equipped torpedoplanes from forward airbases (sometimes situated behind enemy lines) and withdrawing them to rear bases in daylight. That could make the South China Sea a very unpleasant environment to operate in for the Japanese.
But anyway, as long as the Japanese hasn't got Singapore, their campaign is a failure. They entered the war to get a hand on the resources in SEA, and you can't control those without Singapore.
The British feared a surprise invasion of Singapore from the seaside, but the fortifications would make such an operation practically impossible. It would not be funny to be Japanese decision maker, for besides a near-ASB solution like joining the British (like in WWI: if you can't beat them - join them), I can only think of leaving USA alone and focus everything on SEA. The main problem in the short run is that this will probably not free up enough land warfare capacity in SEA to overwhelm a prepared British defence, and operating the main carrier force in the South China Sea in support of the army is likely to grind down Japan's greatest martial asset faster than in OTL.
Next problem is that USA probably will declare war anyway, so FDR had promised Churchill, but it will take several months before the Americans will start crossing the Pacific for serious. If Singapore by that time still is on British hands, the Japanese probably are spending their last drops of fuel. But if Singapore can be taken without inflicting too humiliating losses on USA, I would not rule out that a negotiated peace guaranteeing Japan base rights and strategic supplies could be agreed upon. Certainly not if Hitler also in this ATL has been stupid enough to declare war on USA.
Regards
Steffen Redbeard