Red Dawn I believe was based on a now outdated CIA analysis of possible Soviet attempts to invade the US. At the time John Milnius produced the movie, glasnost and perestroika were starting to come about or had come about. If I recall, Aeroflot flights into Denver were being permitted at the time. Given the GRU had used the old commercial aircraft to deliver Spetsnaz trick in their invasion of Czechoslovakia in 1968, the initial wave of paras coming in commercial airliners is not out of the question. The Sovs were also involved in attempting to spread Communism into Central America at the time (Contras were CIA backed guerillas trying to stop just such a thing).
As for the Chinese invading the US, try Eric Harry’s exciting novel INVASION, about a hypothetical future Chinese invasion. It’s speculation, but a great read nevertheless.