In the first Red Dawn which was set in the 80s. It might have been possible. If the attackers had intimate knowledge of our sat system and was able to mask movements using holes in coverage and such. But it would have taken a huge effort and slowed down their advance to months I remember talk in that movie of slipping up through Mexico and down through Canada and using private airliners and such. But still it would have been near impossible to pull off a total surprise attack.
Today it is totally out of the question. You might sneak a couple hundred troops in at one time but a full fledged battle force won't get within 500 miles of our borders without us knowing about. Hell we will know when the start loading their troops into ships and/or planes at their own military bases. And I am fairly sure our battle doctrine will be to engage these units long before the get near our borders.
Even if the Soviets could have magically transported one or two million soldiers across our borders without being detected, it would not be near enough to prevent them from getting slaugtered by our combined forces which would quickly overwhelm any troop concentrations not supported by planes, tanks, artillery, etc. The whole “Red Dawn” plot is laughably stupid.