By Americanized tacos I mean hard shell tacos with ground beef and shredded vegetables, like those sold at Taco Bell, as opposed to Mexican tacos which are generally marinated meat, onion, cilantro, and a soft corn tortilla.
With the start of Taco Bell.
Glen Bell, the guy who started Taco Bell, was building a franchise system in the 50's to sell Mexican food, and he came up with the idea of frying tortillas to make them less perishable. He claims to have invented the fried taco shell, but there was a patent for fried tortillas in the 40's and similar recipes had been published in a few cookbooks.
The ingredients that go into the shell had been adopted by both the Mexican American community, which worked with what was available, and the foodservice industry, which worked with what could be easily sourced and kept fresh. Much of Bell's menu was borrowed from restaurants in East L.A, but it was Bell's franchise system and fried shell that moved the taco from a food found only in Mexican neighborhoods to one that was available nationwide.
The Smithsonian interviewed Jeffrey M. Pilcher about the taco's history which you can read here. If you want more detail, you should check out his book, "Planet Taco: A Global History of Mexican Food."