This is a way to do it without early war. War with US begins in 1939/40. Forcing the US to split its 1939 army between the northern and southern border will overwhelm it, while still giving Mexico time to stabilise and conquer some territory in the south. Don't puppet US in peace treaty unless you want to have them automatically annexed in the 'redeem atzlan' focus.
I figured this system out myself because I wan't good enough to get Mexico ready in time for 'early war'. I was able to win against the US 6 out of 6 attempts with these rules. You're free to try to switch it up if you want. Hope it helps