Could history be different had the British attacked and taken
over South Thailand in 1941 , thus prevanting the Japanese Invasion
of Singapore?
Stop them - I don't think so. Hold up the Jap advance - yes, to
some extent, until reinforcements could arrive. But what
reinforcements, & how effective could they be? From top-down:
Churchhill was juggling between various Empire needs worldwide;
superior American intelligence didn't always inform Whitehall
(& in any case misread Jap intentions); the Malayan Command
& particularly air logistics were already overstretched; &
the Commonwealth & Indian-Army troops deployed in were of
various quality. Even Force Z was fatally complacent.
The British appreciated the strategic importance of the Kra Isthmus
long before it became a "forward defence" plan under Ops Matador.
They thought that it could be a staging area from which to bomb
India, & also a canal possibly dug out (partly funded by Jap
investments) to strangle S'pore's prosperity. But IMO to actually
invade there, with or without Thai permission, would complicate
matters a lot. Politically, attacking a neutral country would
alienate the Americans. Territorially, the British wouldn't be
after real estate as much as just securing airfields &
potential beachfronts. (Un)Realistically, any such advance to set
up defensive positions must be launched as soon as the Jap fleet
set sail from Indo-China, except they lack adequate intelligence to
time such plans without creating a possible embarassment
instead.
Ultimately, Churchill called the shots &, whether well-advised
or not, he gambled that S'pore could be defended by a local
garrison & sea power. He's said to be very skeptical about the
air-power factor, which Matador relied upon & which ultimately
didn't matter cuz the British had problems protecting all the
airfields in Malaya itself.
Instead, the British fled N. Malaya and allowed the N. Malayan
states to be annexed by Thailand, only to demand the states
(kelantan, perlis etc) back from the Thais after WW2.
Thailand never "annexed" Northern Malaya; it was the Jap
conquerors who pulled the strings. The British "demanded" them back
post-WWII only for their inclusion of the new Federation of Malaya,
but IMO without any insidious motive since they already sensed that
the old colonial relationship could no longer be recreated.
I get you're generally implying, however. Those states were
"rootless" vassals until fairly recently, being Siamese
protectorates until the mid-19th C & then incorporated as
"Unfederated Malay States" within the British sphere of influence
thereafter till WWII.
I also wonder if Malaysia currently has a plan similar to Ops
Matador to prevant the Thais from staking their claims over N
Malaysia?
Those states are firmly under Malaysian sovereignty since then,
& Thailand respects that. Neither side envisions annexing each
other's territories as "forward defence", if that's what you mean.
Doubt the local Muslim population there would accept
Buddhist-Bangkok jurisdication either, since they're intimate with
how alienated their fellow faithfuls in southern Thailand already
feel.
If anything, if you wanna define by old Siamese dominion
boundaries, a bigger claim would be that Thailand owned Cambodia
& Laos more (before losing both to the colonial French).