Roswell, Texas

Makes sense to me. If Texas had never joined the Union, the Mexican War would never have happened, allowing Mexico to remain a regional superpower (absent Santa Ana). The West would have settled differently and the smaller USA would have left a different mark on the world stage. Now, crash an alien spacecraft in Roswell, in far west Texas, amid all these powers and let the fun begin.

This alternate history is best enjoyed with Google or Wiki kept handy, although any history and pop culture enthusiast will have a blast without it! The link starts you on Page One...I killed the whole morning reading the story so far...the forum says it will run into 2008.

H/t to Trainer via Tam.