No, wait. We can do this.
Everyone is assuming that the Thais have to get the nukes by 1945, but that's not the case. They just have to be the first to actually invent them - the first who take the trouble to build the things.
Now, you need maximum Thailand. A glance at Look to the West gives a good example to work from - a Thailand that absorbs Laos and most of Malaya while retaining Thai Cambodia. Ally that with dwarf states in southern Burma and northern Vietnam and you have a major power. Have it luck into the national myth of "holding off all the European Imperialists" as Thande does.
Now we have a couple of centuries to work with. Nukes are expensive - at least until the 1970s or so, when related technologies are more abundant. We'll need smaller industrialized nations, so at some point Russia loses a major war to someone in Central Europe and is reduced to borders like OTL modern, but with Central Asia. The US can be wedged onto a "plague on both your houses" course to isolationist pacifism. At least one, better two, northwest-European powers spend 1-2 generations under ideological regimes that inadvertently wreck educational institutions (see OTL Nazi Germany, or Soviets on a Physics Lysenkoism-equivalent).
Let's have this war - War A - be in the first decade of the 1900s and involve heavy use of civilian bombing, and almost entirely by the losing power. That means we need the steamplane for earlier heavier-than-air flight, or an outright airship timeline. We want this so that people become horrified by civilian fatalities. Better have someone use chemical or biological weapons, just to be sure. This might be the war that splits up Russia. A second, smaller war - War B - will involve much of Europe gang-piling an aggressive rogue state which violates the new rules of war and in failing pathetically, serves as a morality tale. This one is good for involving weird ideologies that cripple nastier states that might seek nukes.
This timeline's photos of bombed-out cities are not dime-a-dozen shots from half of Eurasia. Instead, they are the equivalent of death-camp photos in OTL. The sort of thing you show kids in high school when they're "old enough" to learn about human evil. Essentially, by the 1920s the modern attitude toward civilian fatalities has been reached. Then there's one more war featuring a desperate state breaking the taboo and everyone else outweighing them so extensively they don't need to. That cements world consensus against city bombing, bombing of civilian areas, or even major artillery in the same role. By the time the '60s come around attitudes are stronger than OTL 2010 - the thought of bombing something within a quarter mile of a hospital would spark universal outrage.
Nuclear power is invented in Europe or America, and becomes widespread in both, though more quickly in the latter. By the time scientists point out the potential of The Bomb, most of the pressing territorial issues of the world's major states must be resolved. As such everyone is willing to abide by treaty regulations against bombs that cannot be limited to military personnel. Russia starts a nuclear program at one point, but it loses funding and stalls with a governmental coup. It's fine for Armenia and Azerbaijan to be at each other's throats, Africa to be a huge mess, and so on. But China, Japan, major European states, et cetera must be broadly holding all of their percieved core territories. The same goes for India, though division into a 8-10 hostile states would also do the trick.
While all this is going on it is conceivable that Thailand-and-Friends can conquer southeastern Indochina (modern east- and central-Cambodia and south-Vietnam), most of Burma, and the rest of Malaya. Throw in a division of northern and western Indonesia into "liberated" states that will be under Thai "leadership." It probably splits Vietnam and Burmese acquisitions with it's "allies." Either way, most of this expansion takes place during War A, and Gross-Thailand hops in on the winning side well after the likely winner is apparent. [Just make sure the Europeans across the border turn out to be on the losing side.] Such times were one of the few where European powers would accept native conquest of European colonies and protectorates.
Thailand goes all out on attempting to replicate Western education and getting in near the start of the spread of Nuclear Power. It makes progress, but is much more successful at the latter (the West still dominates higher education for a reason in OTL). At some point, something goes wrong. Thailand loses influence in the East Indies and is diplomatically isolated. Maybe Thailand takes the wrong side in War B, maybe it gets a weird ideology, maybe India and China get matching ideologies and Thailand overreacts. Whatever, Thailand threatens its neighbors and makes a pariah of itself internationally.
In the mid-1970s, Thailand's defiant stance is looking increasingly unwise. It is certainly a titan by the standards of the turn of the century, but is losing ground in energy, computing, space, chemical, and biotechnology. It decides on the perennial course of ideologues - a great leap forward. Plans are stolen from Russia's aborted project and a few somewhat-competent, starving, and morally indifferent physicists are hired to supplement Thailand's own. Sanctions imposed by the outside world halt outright production, but research, testing, and designs continue.
In 1995, an overreaction to a border incident with China puts the Thai bloc back on the world's shit list, and the nuclear program resumes outright production. On October 3, 2006 to the horror of the world, Thailand becomes the world's first nuclear power.
Gah....
Next time ask me to do something easy, like Sealion. I feel drained.
Thoughts?