Most national dishes are said to have simply evolved, cooked into being over centuries by anonymous hands. Pad thai has a paper trail instead, because it was substantially the work of one government trying to invent a nation.
In 1939, Field Marshal Plaek Phibunsongkhram, then in the first of what would be two terms as prime minister, renamed the country from Siam to Thailand, part of a wider program to fuse a modern national identity out of a kingdom that had never needed one before, having never been colonized and so never having had to define itself against a colonizer. The name change was cultural policy, not paperwork, and it came with a companion campaign aimed at ordinary daily life: a set of state mandates called the Ratthaniyom, the Cultural Mandates, that told Thais what to wear, how to greet each other, and, eventually, what to eat.
Rice noodles were part of the plan for a specific, unglamorous reason: Thailand was exporting a large share of its rice crop, including during a period of wartime shortage in the early 1940s, and Phibunsongkhram's government wanted citizens eating less of the grain directly and more of a dish that used less rice per portion while still feeling substantial. Noodle-based dishes fit the goal, and the government did not merely permit them, it promoted a specific one. State radio broadcasts, government pamphlets, and even instructional recipes distributed under the campaign held up a stir-fried rice noodle dish, built on tamarind, fish sauce, palm sugar and dried shrimp, as a model of modern, efficient, patriotic eating. Street vendors were encouraged, sometimes with direct subsidy, to sell it, and a name adhered to the promotion: pad thai, literally "Thai stir-fry", a name that asserted a national ownership the dish had not obviously earned yet on its own.
The irony sits close to the surface. Stir-fried rice noodle dishes were already common across Chinese immigrant communities in Thailand and the wider region before the 1930s; what the Phibunsongkhram government did was take an existing, ethnically Chinese-inflected cooking style, standardize a version of it, attach a nationalist name, and promote it hard enough that within a generation "Thai" was the only adjective anyone associated with it. By the time pad thai became one of the country's best-known culinary exports in the late twentieth century, its manufactured origin had mostly been forgotten by the people eating it, in Thailand and abroad alike. The dish is not fraudulent for having been promoted rather than simply discovered; every cuisine absorbs and reworks what crosses its borders. What is unusual is how well documented the moment of absorption happens to be, down to the government office that pushed it.