An Attack On The U.S. By Mexicans

The Boiling Frog Syndrome: "If A Frog Is Placed In Boiling Water, It Will Jump Out, But If It Is Placed In Cold Water That Is Slowly Heated, It Will Not Perceive The Danger And Will Be Cooked To Death." - Wikipedia
Or
"If The Camel Once Gets His Nose In The Tent, His Body Will Soon Follow." - Wikipedia
See Also: Slippery slope, Domino Effect, Foot In The Door, For Want Of A Nail, Creeping Normalcy