Sorry Peg, we would send troops in a blink of an eye in Mexico if it were to cause instability to the USA, you yourself should not be so naive.
Remember the Bay of Pigs? Remember the Falkland Islands (British) w/American encouragement. Remember the Panama Canal and Ortega? On and on the mighty USA does not hesitate to act on anything in its hemisphere.
Naïve? I'd say I'm quite realistic at what this country would and would not do.
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