Thanks for all of the input, guys. You've helped me come up with something that I think approaches plausibility.
How about this? Starting in 2015, in response to Russian annexation of Eastern Ukraine (I think it's safe to say now that the writing is on the wall), the U.S. negotiates permanent military base deals with Poland and the Baltic republics. As a tit-for-tat response, the Soviets negotiate base deals in Cuba, Venezuela, and Nicaragua, starting later that year. That place as least some Russian military elements in the western hemisphere not too too far from the United States.
Later, as war breaks out in Asia between China and the U.S., prior to a planned Russian invasion of the Baltics, the Russians convince Mexico to take advantage of America's overextension overseas by attempting to regain the American southwest by force. The Russians provide direct military support- a new "Division Cuba", if you will- to the Mexicans. I bet that the Russians could cobble together at least a division from their personnel presited on their Latin American bases.
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