This question already has an answer here:
I'm from Mexico and planning a trip to Japan, and so me is a few thousand dollars cheaper and way faster to do it from LAX, than from my own country. I do have an American Visa B-2 type (Tourist). What problems could there be regarding my situation? and Are there any extra steps in me taking an international flight within the US as a foreigner? Thank you so much in advance, I really don't want to have an issue and have never done a flight like so.