Yeah, like U.S. intelligence would never intercept gigantic movements of Soviet troops and materiel numbering in the millions of persons and tonnage. Utterly preposterous. Any attempted takeover of Mexico by the Soviet Union in the eighties, and with Reagan as president, would have been met with proportional force. Absolutely no way.
In the 80's, sure. The premise at the beginning of the first movie is that the dominoes had been falling for years.
Look at Venezuela. See any action at overturning that dictatorship? Look at our own government -- any sign of serious roll-back of socialist measures? Look at Europe -- see any sign of mass deportation of Muslims?
At each stage, an incremental victory for the Communists would be achieved, with them threatening total war at any attempt at roll back. Eventually they would be close enough.