So I'm curious if there are any solid possibilities for having the Japanese assault in 1941 fail? To narrow the parameters lets say the earliest you can make a change is June 1941, and that 'replace MacArthur' isn't not an answer in itself. My own reading is that much like Malaya the Philippines was at the bottom of the priority list for equipment and manpower. MacArthur's attitudes and pomposity might not help but I think you would have to have the islands get a higher priority for men and equipment. So any thoughts or suggestions?
The USN had pretty much written off 'forward defending' the Philippines in any then future confrontation with Japan and instead was committed to the 'Mandates' plan of island hopping as they did after watchtower.
It was almost certainly the correct choice as the loss of the Philippines would not have been a deathblow to the USA (and so it proved) but heavy losses to a forward deployed USN might have been in the short term.
It is similar to the RNs plan of resorting to a distant blockade of Germany in WW1 and not the expected close blockade which was disappointing to the Germans as they had based their strategy over the British doing just that.
So we can not expect the USN to save them (and even with out the losses of Pearl Harbour given the disproportionate losses in surface actions between the USN and IJN in many of the surface actions of the pacific war probably a good thing).
What could be done is an earlier expansion of the Philippines army from less than a division (effectively a Militia police force) to a 10 Division force far earlier and the equipment sent far earlier.
Particularly heavier equipment - Artillery, AAA, command vehicles, mobile STD clinic etc
But therein lies the riddle - why?
When would the US decide to stand up a Philippines Army many many times the strength of the peacetime force - for me there is no real reason to start this process before 1939 and arguably not before the partial Japanese invasion of FIC in July 1940 (which at the time was only really intended to block a major source of arms to the Chinese) and far more likely the subsequent total occupation of the French Colony in mid 41.
It does not give those planning such a force a lot of time and so it proved with all 9 of the planned divisions each only having one of its 3 Regiments stood up before 7th Dec 1941 with the planned 2nd regiments in each called up for training as the Japanese attacked and the 3rd if at all while the campaign was under way.
Most of the equipment planned for this army was still in US Warehouses stateside come April 1942.
It was the same with the air force stood up in the Philippines - a lot of what had arrived had only arrived recently and was pretty green.
And there was a large burden on US production for the at war Democracies in 1939-41 and while the USA did become the arsenal of democracy etc it was not yet that in this period - the UK still was and was struggling to supply enough stuff for its own needs and a lot of what teh USA was producing was for its own expanding military with a great deal of kit going to the British who were actually fighting.
So low hanging fruit then.....
An earlier conscription effort with better arms and equipment ordered earlier - with perhaps a more realistic mobilisation plan (not the unworkable nothing to 10 divisions ASAP) that allows Staff and field Officers to be educated and NCOs to be discovered and gain experience etc.
This process takes time but even an earlier limited conscription would pay dividends to a later larger call up if it can be managed as there would be a far large 'estate' of educated officers and NCOs than was the case OTL
The air force perhaps organise the airfield better - I mean just look at what the British were doing with their bases - revetments and dispersed and often camouflaged deployment of aircraft with satellite airfield and even decoy sites.
A chain home system and fighter command system is almost certainly beyond the Philippines during this time but a system of coast watchers with a centralised command structure to control everything.
And leadership - I am not sure swapping Big Mac for anyone else is going to make things better.
He was well known and trusted within the Philippines elite, and I understand that he was something of a poster boy for the Republicans and Southern Democrats back in the USA so would have the connections to get stuff moving etc that another choice might not have.
So he even though we all hate on him, and while his military leadership was found to be seriously lacking he is probably the correct choice certainly to start an earlier building of a Philippines military.
Lastly....racism - the powers that be have to accept earlier that the Philippines can be good soldiers and appreciate earlier that the Japanese are peer opposition and that their equipment is as good as their own.
It is probably the hardest thing to fix.